Would 54"*3yards or carpeting be enough to make a custom box for 2 12" Infinity Subs?

 

should easily be enough. my 3 cu ft box was roughly 36x18x12 probably about the same size as you'll have...
I used under 2 yards to carpet that box...

I think I bought 6 yards total, carpeted the box and entire bottom of the trunk after I dynamatted it, and still had 1-2 yards left.

besides, at about $10/yard, the stuff's cheap enough to throw away if you buy extra!

 

aight thanks