Cropped top ... How bad can it really be?

As some of you know, I am using the time between now and spring to research options on a sub project!
Cubos are strong front runners at the moment,but I am still looking to be sure!

I am currently also looking at different ways to utilize this driver www.pyleaudio.com/manuals/PDW21250.pdf .
Unfortunately its EBP is to low to consider a Cubo variation.
I may actually buy a single driver to start with and make a really simple ,cheap cabinet just to be up and running.
Definetly with consideration to buy another driver and make better cabinets later.

Anyway I have been running it through BassBox Pro 6, but Converting my volumes to trapezoidal sphere (traps reduce standing waves). While doing so I notice thaT I could crop the top and make the cabinet front 24"x24" and could make two complete cabinets for a 21' sub out of a single sheet of 4'x8' material (plus bracing of course).

Bassbox pro shows peak output for a ported cabinet using a boxvolume of 3.49 cu ft ( volume taken up by driver is not yet accounted for BTW) and High fidelety with a volume of 10.15 cu ft.
Of course BB pro is considering these volumes with a rectangular prizm. My square front trapezoidal prizm has an aproximate box volume of 5.33 cu ft.
My question being is just how bad could they really be? I mean because of the shortened top that is.
EDit:
I realize that cabinets that are slightly taller than they are wide reduce standing waves,but won't the trapezoid shape help offset this a bit?
2nd Edit:

Would going to a heavily dampened internal invironment help ?

The driver wants a vast cabinet.

Try looking at some other 21" drivers, for example the Precision Devices PD2150 has more BL and can therefore work in a smaller box.

On the other hand, if you want 98dB to 25Hz, you'll need a vast cabinet no matter what.

One way to do a much smaller cabinet than the driver wants is to do 3rd order passive assisted. Basically a sealed box and you put a capacitor in series with the driver. It's documented in Vance Dickason's Loudspeaker Design Cookbook. You would need lots of unpolarised electrolytics to get your required capacitance and voltage rating though.