I made a false floor for my old Prelude.
Used the factory carpet and traced around it on 2 pieces of mdf.
Cut it outand test fit, marked any spots that wouldn't clear and took off little bitsat a time till it all fit nicely.
Haven't looked in my Skyline boot in a little while but with the Prelude the tyre sat up a bit into the boot so I used some mdf cutoffs and stacked them to bring it up to the same height then liquid nails & actual nails them on as little feet.