... kinda hard to answer that... i mean, you mount the amp, then make some risers that are level to each other, running wide so they support the longest way possible, you want like a plexi glass face, or like take a picture of the space and another pic of a simular one to what you want. i mean, mirrors, led's, hydrolic lifts? i mean, there are like 50 thousand ways to do this...
k so this is kinda like what you are going to want to do exept that your top peice is going to be a lot smaller, if it's the back seat of a truck.
um, just a question... but what tools do you have access to? table saw, router, jig saw, skill saw, sander, screw gun, nail gun, grinder with a sander pad, not like this is a small job, and what experiance do you have? your dad? lol i think your best bet, would be laying 3/4 on the ground, putting your amp on that. cut some blocks so the top cover wont hit your amp. then make a top peice that will fit in the hole of the floor pan. maybe run some screws strait throught the top of your cover into your support blocks so it doesn't move. box in your amp so the top supports dont buckle, or cut a few corner blocks to stablize. unless your dad is heavily into wood working, simpler is better... bare minimum is a jigsaw, drill, and yellow glue. sand the edges you cut with a block sander. everything should be semi smooth so the mdf doesn't cut your carpet. i've slit my hand open on a 45 with med dens. pre drill everything, or your just gonna blow the wood apart. the carpet will give you an 1/8th of fudge room. a custom scribe is not a one shot deal. it's gonna take probably 20 passes for you. scribe the back, based on whats strait in the front. then scribe the left side based on the back then the left side. 1/8th gap is fine. a custom design is based on what you want and your ideas. there isn't a set blue print of what it needs to look like, or any measurements that could help you beyond the height of the amp. if you want actuall construction help, supply an idea of what you are looking for.