Yea, kind of like a false floor. Just raises everything up, so when you rest the amp on the original boot floor (maybe with some perspex over it?) it looks sunk in. You could even buy a single neon strip, or something along those lines, and have it lined up next to the amp also sunk into the floor, just to give that extra glow. I think Neons strips have gone out of fashion though now.
As Joe said, make sure the subs are enclosed. The enclosure improves the overall sound, and adds that Bassy kick that they provide. otherwise they just become a rattly speaker.
I dont have an enclosure in mine, just the sub in a standard box, and the amp secured down next to it. Ive kept it though, so if I need to change a wheel (puncture etc) it can all be disconnected and lifted out, allowing you to get to the wheel. Where are the spare wheels kept on an Astra??
If you want to spend a little bit extra, or have knowledge yourself in Composites, you could make a Fibreglass enclosure. Would end up being lighter overall.
I dare you to make a Carbon Fibre/Kevlar mixed composite Enclosure