Im looking for some adviced from people who know.

Ive got dual 12" subs in original box - Kicker CompVR.
Ive been told that if i take em out of the box i will never get the same sound as its original box... but thats ok cos the box is massive and im willing to sacrafice sound quality over boot space at this point in my life.

I want to build a slanted box so even though the box is still taking up space i can lay stuff ontop when i need that space.

Will too deep a slant decrease quality?
Should the box be sealed or ported for a quality note and reletive impact?
How thick MDF?

Anything else you guys think will help me will be much appreciated.

if the box is sealed, it doesnt matter what shape it is, as long as the internal volume is the same, it will sound the same.
even ported generally isnt majotly affected by shape, though it does play a part.

as for never sounding as good as the original box, thats a load of crap :) all comes down to preference, you might think ported sounds better, or dealed, so they cant just say 'this sounds best'

Kicker's pre fab boxes are designed to be the optimal internal volume for that sub. if its a sealed enclosure, like Woob says, keep the internal volume the same and it sound sound the same as the original box.

just remember though, the manual you got with the subs will have a guide with the minimum volume - maximum volume recomended. the bigger the box within that recomended range, the better SQ.