If you want to secure this down then I personally would get a thin bit of mdf (1/4"-1/2") to roughly fit and fill the floor area of the boot and then bolt the box to that.
The trick with the bolts, would be to place 4 in a square shape permanately fixed to the mdf above the spare wheel well (so the bolt comes up for you to then screw the nut on). This way the wood will still fit flush and if you only need to unbolt 4 nuts to remove the box and that thickness of mdf won't weigh much either.
Agreed, get a sheet of MDF and use your boot carpet as a template to draw round and then cut the wood with a jigsaw (an electric saw)
Also a couple of bits of 1" x 1" bits of wood attached/screwed to the underneath of the MDF going from back of car to front of car (lengthways, not widthways) to allow your speaker/ amp cables etc unde the fake floor.
And then you can place the Mdf Fake boot floor in your cars boot with the boot carpet on top of it and you'll never know difference (apart from the boot being a few inches short depth wise) between that and the normal floor.
Then screw your sub to this False MDF floor with 90 degree right angle brackets
And if you really wanted you could drill a few holes and cut a line in the boot carpet above the holes in your MDF floor to let you amp cable dissapear under the floor out of sight to where it goes (the headunit or amplifier) and if you have an amplifier you can make the speaker, RCA(phono type audio leads), and power and earth wires dissapear under the floor to the headunit, Earthing point and Power supply
The only thing about having a full size MDF boot floor is you'll have to ditch the spare wheel. or cut a square out the MDF and put on a hinge on it to access spare wheel. But you may have to screw amp onto the back of seats to do this if you have one.
And have sub to the left side of boot against rear left wheels arch.
Or the other option is to cut a bit of wood big enough to sit over spare wheel area(As Terry172 mentioned) and do the same as full size false floor above but on a smaler scale
And now i'll apologise for my long reply.
