I made one last week for my kids on a piece of mahogany (had it lying around, not necessary). I used a 185g tuna tin, upside down with a small wedge of wood on top in the middle to act as the "bridge". It's not fixed there, it is just pinned by string pressure. It effectively gives you a resonator effect... kind of like a dobro. to hold it all in place, I routed out the wood to make a channel for the tin. You don't need to do this. nails/screws placed nicely will give it something to lean on instead. I drilled a hole in the bottom (10mm) to "let the sound out".
The result: Wow, this thing is loud! I jammed an old humbucker on there so I can amp it, but it's really good acoustically. I was amazed at how much clear volume I got out of it.
A really cheap, and easy build that works well.
See my attached photo