You might be able to go with an unlined fretless and a long body. Calculating fret placement is easy enough. You could get someone to fret it for you or for that matter have someone build you a custom neck. I've been thinking of building a 36" scale. I'm curious now as to how many fret positions would be lost.... do you need 20 & 21? Looks like i'll be surfing the warmouth site to see what there longest body is. It would be a cool project.
I did some calculations based on warmoth parts and I don't think your gonna get out to 40"'s. .....
Now you got me thinking about my next build..... I have a neck blank and a long piece of mahogany. I measured up last night and I think I could pull off a 40" scale bass. It would double as an electric upright. I just need to look at available truss rods or make my own.
Go to the hardware store, get some lumber, make a narrow table as long as you wish (6'?). Buy some length of thick metal cable, like they suspend bridges with. Use some imagination after that and you'll have a fine low-frequency generator.