Back in the late 90's I had a bis with two 10's in a box which was built in to where the back seats should have been......... There was more power in the ICE system than the engine lol
But a smaller removable box with an 8 inch can be made to sit in the boot. Just fit some handles and a quick release banana plugs and multi connector for the amp and sub so it can be lifted in and out easy for access to the engine