I mocked up the measurements from my research to see if things were accurate. Some stuff was off and I got the idea to have the front be a facade like they used to do to shacks in the wild west. The outside was just a flat, cheap wood wall painted to look like a palace and the inside was unfinished lath wood. So, I'll use 13 or 14 straight 1/2" x 4" pine lath pieces for the sides roughly following the instrument's contour but in straight lines of course. They'll be connected with 3 1/2" piano hinges or some such. I'll use 1/4" hardboard/Masonite for the top and back although I imagine the sound would be better if I used hardboard for the sides also. I'll use a double bit hickory replacement ax handle for the neck using "through body" construction. The finger board will use 5 1/2 inch diameter plastic pvc or abs conduit cut at an angle lengthwise so the arch's width is thinner near the nut and wider toward the bridge. I'll cut and shave down the end of the axle handle to create the correct string angle over the nut and use wooden viola tuning pegs from the music store.
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I like your design.
The sides will be fine as long as the wood is sturdy enough.
The top and back vibrations is where it's all at. Birch has good tonal properties and if you can find Lauan (asian Mahogany) is very good. Red Oak is good, but can be bright. I have seen Spruce plywood in some Lowe's stores.
I found some Sledge Hammer handles made of Hickory that are straight, 3 foot long an about 1&1/2" to 1&3/4" diameter. Great for necks, although a 6 string may need something bigger. Stairwell safety rails might get you what you need.
Sorry I wasn't more clear, I meant your wood choice for the side will be fine as long as it's sturdy because the sides don't offer much for tone. Tone comes from mostly the top and some from the back.
I would look at some acoustic guitar internal pics for good bracing techniques for the top, back and sides.
A center block is a bit of a catch 22 situation. It robs you of some volume, but adds sustain. I have a ES330 clone guitar with a 1"x1" center block and kerfing around the edges of top and back. Top, back and sides(3/16th" thick) are plywood with no other bracing. It's held up very good since 1965. Doesn't have a lot of acoustic volume, but it sustains very well. The thinner diameter you use for the center block, the more added volume you'll get. You just have to find the right measurements for the block.