Tyeler, hello...
Technically, sure it's possible.
But you'd really want to consider building it on an empty tank if you want to go full 3D including overflows and the sides of the tank.
You could make segments, attach them to bottom plates to hold them down (assuming you'll use styrofoam/concrete/epoxy), then empty out the gravel, put in all the segments of the background, put the gravel in again (on top of the bottom-plates) and you're done.
Once more, it totally depends on how the background will have to look to make you happy with it. Mine is permanently fixed inside the tank, which I think is the best way to have a complete and most realistic look. No gaps, seams and other details that look bad.
Initially, it looks quite fake, but over time, your background will get covered with algea and other things.
Here's a picture of the background I made, this is 3 years now I think without any problems....the straight horizontal line, green below and brownish above is because I lowered the water-level for some time as I make changes to sump. within a couple of days it grows and colors back again...
the tank is 150cm long, 50cm high, 50cm deep. Only 5 cm max lost in certain places to the background. mostly it's 2-3cm
I fully agree that the ready made backgrounds look nice, but for me there's nothing that beats doing it yourself and you'll be much happier with it.