Robster - I was thinking about this last night...
I took the back off one day just to hear what it sounded like without it. Personally I thought it sounded like garbage. Too open and airy. No character. I put it back immediately. BUT try this...
Get some 1/4" plywood and cut it the same as the back...drill the screw holes etc and then drill or cut out a chunk of plywood to use as a port. Or just use 2 pieces of plywood cut so that one sits on top of the other and one is at the bottom, similar to other "combo" type amps out there. Hack on some cheap plywood to see if you like the results. If you do then cut your existing back the same, or just cover the new pieces with black tolex material.
Guitars/Bass - MIM Fender Classic 50s Strat, MIM Fender Standard Strat, Squier Classic Vibe 50s Tele, Gibson Les Paul Studio, Epi '56 Gold Top Les Paul, Martin DSR acoustic, Sigma Martin Auditorium electric/acoustic, Squier Jazz Bass.
Amps/Cabinets/Modelers - Model 2558 50 watt Marshall Silver Anniversary Jubilee combo w/ Celestion Vintage 30s, 4x12 Marshall cabinet w/25 watt Greenback Celestions, Fender Blues Junior w/ a couple of Billm mods, Line 6 POD 2.0, Roland Micro Cube
Pedals/Effects - Cry Baby Classic Wah, Boss TU-2, Boss NS-2, Boss RC-2 Loop Station, Ross Compressor, MXR Micro Amp, Danelectro FAB Echo, Danelectro FAB Chorus, Danelectro Chicken Salad, Marshall Guv'nor Plus, Marshall Echohead, Duhvoodooman's Zonkin' Yellow Screamer, Digitech Digiverb, Digitech Bad Monkey, Dunlop Fuzz Face, Homemade Loop Bypass pedal, Duhvoodooman's Sonic Tonic (Maxon SD-9 clone +), Voodoo Labs Superfuzz