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Thank you for your suggestions but I don't think I want f-holes on this instrument. For me it's visually paired to a body with a waist. But I'm looking at historical instruments for inspiration. Old viols, citoles, vihuelas and whatever they were called have all kinds of sound hole designs. And I might end up with f-holes anyway!
Rob: That's a nice looking arch-top! I've seen images and some movie-clip with Nkforsters cylinder-top instruments and they look/sound very good and insteresting. Thanks for giving me your "source".

And here is my original idea of sound hole:

I'd really love this design which combines sound hole and sound port. But I never found out how to compensate for the weakening of top/rim-joint so I gave it up. But if anyone have suggestion I'd be glad to reconsider.

So now I'll start the next project. It's a mandolin which share many features with the newly completed tenor mandola - same body shape, same materials and very similar design principles. But just like I stole a lot of ideas from you guys in that project I will continue to ravage and pillage like a true viking. This time I've been inspired by R the F-s experiments with Howe-Ormish design.
But I did my own research and found that many of these ideas with cylindrical tops, floating buttresses etc were partly invented by swedes emigrating to the USA at the end of the 19th century like the Larson brothers and the Vega founders. So I hope we can all agree that I'm just reclaiming my heritage :-)

"Double-S" is just my word for the "bulge" or "hump" that were on the Howe-Orme and early Vega guitars (and on R the F-s guitars of course). Traditionally they've been called "cylindrical" tops, but that's a strange thing to call it. My tenor mandola and many other instruments are true cylinder tops as they have a profile that is a part of a circle's circumference. But here we have a profile that is more like a double s-curve, hence the name.







I'm still concidering two alternative tops - either I'll use a traditional centered round hole or I'll use two or more smaller sound holes placed off center. Not necessarily as on the picture but something in that direction.
The reason why I concider the version to the left is mainly structural: I want to try out the "double s" because it gives me the strength exactly were I need it - that is along the line were the strings try to bend the top and were the bridge tries to push it down, while the rest of the top hopefully can be left without so much tension and concentrate on vibrating and making good sound.  It seems like a bad idea to cut a big hole just were I want the top to be the strongest. Lets just say that it's always a bad thing to make a big hole in a vault construction as it is the continuity of the vault that gives it its strength.
The reasons why I nevertheless might make it with a traditional sound hole is partly estetically and partly practically - I like the traditional look and I would very much like to have a bolt-on neck that I can remove and adjust which requires a hole in that position.

The bracing will be a little bit different depending on which version I choose, but the principles are the same: Ladder bracing combined with carbon fibre floating buttresses:



The round hole version has  A-braces to strengthen the area around the sound hole.



Here is a version of the side hole top with a pure ladder bracing. But I should probably put some longitudinal braces near the sound holes.

Some drawing views:







But the drawing is just a design aid - the final instrument will develop in the building process. I'd be very happy for any comments and suggestions
Tomorrow I'll sharpen my smoothing plane and start working on the top. But it will probably be a slow build as I have far too much work and too little time for this.

Riverman: I think the dreadnought shape were drawn by the wood supplier. I don't really know why, but the top and back plates often comes with a guitar outline drawn on one of the sides. A few images further down you can see the shape Ashley has drawn on the same piece of wood.
Ashley: really beautiful Sapele and I'm sure it will be a beautiful instrument.

I still have some minor adjustements to make. May be lower the bridge a bit, may be re-cut the string slots as there are some sitar-like sounds from the treble strings. But over all it's ready.
Most of all it's a very good mandolin when capoed on the seventh fret, with long sustain, high volume and a crisp, clear sound. When played as a tenor mandola it has a sound geared towards the treble register. I think I like it, but I'll have to get used to it.
If possible I'd like to lower the action a bit.
All in all I'm very happy with the outcome. I'm a bit nervous about the fact that I've built in quite a lot of tension in the instrument. The break angel of the strings is higher than normal and that is what gives it the volume and the overtones. I hope the bracing and the carbon fibre rods will make it!

In the original design I had planned for an x-braced body. But problems with other instruments that I built earlier made me search for alternative solutions. That was why I ended up on this forum. So I changed to a cylindrical top and added carbon fibres inspired by Dave White. Lots of credits to him if I succeed and no shadow on him if I fail!

Some of the steps in the making of the fretboard














And here it is glued to the neck:

Glueing kerfed linings:



And here it's completed:



Glueing the top:



And the body is closed: