Home Theater Subwoofers
Stereo Integrity used to sell a really great alternative subwoofer (HT18) to the Ultimax 18. However, they did not sell well enough, even at their bargain of a price. So the owner discontinued them. If you can find some used for cheap, then I would say go for it. If I recall correctly, they were cheaper than the Ultimax to begin with, but I don't think you'll find many out there anymore.
Here's a possibility:
Dayton Audio RSS265HF-8 10" Reference HF Subwoofer 8 Ohm from Parts Express and Yung SD300-6 300W Class D Subwoofer Plate Amplifier Module with 6dB at 30 Hz, also from Parts Express. Here's an optimized box.
Maybe make a reading bench out of it?
The 3" port was just a quick eyeball measure to keep velocity down. Smaller ports will be shorter, just be realistic with the amount of air the woofer can move.