i made a false floor in 4 parts, covered in acoustic material, and the sub was screwed sideways from the floor, the parts of the floor could be pulled up very wasily around the spare wheel to get it in and out etc, but once the floor was it it looked impossible to get out because of my smart planning...lol
definatley secure it down, dont take chances with that, you never know what could happen, i spent one saturday afternoon doing mine, well worth it.
also as said make sure you have the tools you put it in with to take it out if u get a flatty, just plan it properly before hand and youll have no problems.
heres mine....