If it's a custom box to fit a specific vehicle then I would say it is worth building if there isn't anything available that will work for you. One the other hand if you need a simple box there are tons of them on ebay for the same price as purchasing the materials. For instance my friend was going to build a new box for his wifes 2 12" L7's. The wood, cloth,spray adhesive, glue and screws totaled $42 and some change. He got a dual box off ebay brand new for about $48 including shipping. It is already built and carpeted with speaker terminal cups ready to go. I recently experimented with a design for 03 Dakota CC that was essentially a take off of a Thunderforms model. The problem was I already had 2 10" L7's and they Thunderforms only fit 2 10's with a rather shallow mounting depth. I had a bunch of 3/4" plywood laying around so I mocked it up and installed it. I painted it and it looks like @ss and it rattles like an SOB, but like I said before it was only an experiment. I try not to let anyone see it. It looks like crap. A word of advice DO NOT PAINT and DO NOT USE REGULAR PLYWOOD. I am also thinking about getting some new subs and go with round ones instead of square. I really like them, but the JL subs are looking pretty good.
yeh of course youre not sopposed to use plywood. IT flexes :-) Use mdf wood.