I build boxes, too, and I have to tell you that if there was a market, you'd have the problem of charging an amount people would pay, and yet enough that it'* worth your time, tools, and supplies.
If you were doing, let'* say, 5 or more boxes at a given time every week, you might be able to save some of your time in the shop and make it worth while, but doing one here and there, and only making like $20 bucks profit might not cut it.
As with anything, it'* worth a try, especially if people are willing to make it worth your time!!!
'95 SSEi, few electronic mods (and a stupid coupler)
HU (Sony MDX-CA680, European Ver; Sony 4x3-ways; 91, 93dB)
Yup, that'* ISU