Help choosing a sub for my odd box ?

Which sub should I go with ? My budget goes up to $300, I have a kenwood amp I am going to be running at 400 rms, but the real problem is my box... I couldn't decide sealed or ported box so my friend talked me into getting this bandpass box. There is no problem with the box only that I can't get my sub in there. The hole is 12" but the box has the insert for plexiglas which is obsructing where I put the sub in so the sub doesn't really fit !!! I can't return the box so I'm using it for sure, so I was thinking about doing a kicker cvt for tight spaces and twisting it in there but I don't want it to sound bad. Is there a way I can modify the box to get a normal 12 in there ? Pleeeeease help me !!! I'll see if I can get a link to my box if it helps but any help is appreciated not dumb comments tho thanks guys !!!