I'm sure you could find someone on here to build a box for you. It'd probably be cheaper then the ones at Best Buy and it would also be a lot better.
Those Type R's that were mentioned earlier in the thread have dual 4 ohm voice coils. All subs have voice coils, some are single while others are dual and some come in quad coil configuration. Since the Type R's were dual 4 ohm, each individual sub can either be wired in parallel to 2 ohms each or in series to 8 ohms each. It depends on the amp you choose as to how you'll need to wire them. If you wire them at 2 ohms each, then you'll need a 1-ohm stable amp. If you wire them to 8 ohms each, then you'll need an amp that'll work at 4-ohms. It will probably be cheaper to find a 1-ohm stable amp to power them then to try to find a 4-ohm amp that will produce the same amount of power.
You might want to read here:
http://www.bcae1.com