corner traps-

I'm using Certainteed CB-350- most all the plans I see use the full 24" dimension of the rigid fiberglass in their corner traps. Is this because this dimension works best? ... or it's convenient to not have to cut the panels down? Is there any advantage to cutting isoceles triangle wedges and layering them to form a solid corner wedge? or is the air space behind critical? why am I asking? I don't have quite enough room to use a full panel... and if I have to cut it down why not just make a solid wedge? according to the room mode calcs (and numerous critiques of my mixes) ~225 Hz (and multiples) are the frequencies of concern... especially ~900 Hz Thanks Stainless Sacramento