took back seats out, need more room Options

[FSX_Celica94GT]

so i took my back seats out over the weekend cuz i need more room to put stuff since i have a giant sub box in the back of my car. I need more room for my back pack and computer when i go to lan parties (noone ever sits back there anyways), but my question is, since theres absolutely no carpet underneath the seats and theres 2 dips in the back where the seats were, i was wondering what would be the best option to fill this in with? i was thinkin about gettin some carpet and just putting it back there, but then i decided i was gonna move my subs up so i can have an easier time getting my backpack out. Maybe a thick board cut to fit the back would be best? what do yall think?