The first step to building something like a 5x7 back is to ask if anyone has one they want to sell. I like working in wood as well as anyone but life's too short to re-engineer something that was done many times over by people much more talented than me.

B&J 5x7 Grover back in gray. 20 bucks plus shipping. You'll have to refinish it, modify it to your purpose but the hard part was done 50 years ago.

tim in san jose