limited space and i want some better bass

i have a rockford fosgate 15 inch punch z sub atm with an alpine mrd-m500 amp. I have 2 major probloms. Problom one is i have limited trunk space so i got a 2 cubic foot box for 15 inch sub and also the sub was a 4 ohm dvc sub but a voice coil stopped working. Im fine with only having 1 4 ohm voice coil working but my amp puts out more power when ran at 2 ohms. So my questions are what would be a decent size box for this sub (i have a hyundia elantra and i need to keep some room in my trunk) and is there any way i can get this sub down to 2 ohms?

Look at some 12 inch subs. 1 12 in a proper sized box will sound better that 1 15 in a box thats too small. Theres no way to get the sub down to 2 ohms with only one coil workinUNLABELED Custom Car Club President