If I remember right, I used 12/4 for my wiring from my KBAC to my motor, as it's what I already had on hand. I think I had some 12/3 for the power in as well, but you could also just use a 12/4 and cut out the white conductor. To get it to fit, you can separate the strands into 2 "bundles" and twist them together as tight as you can, then put a bundle on either side of the terminal screw before you tighten the little clamping plate down. It helps to strip more wire than you'll need, before you separate and twist, then cut to the length necessary. You can also wick a little bit of solder in to help keep the strands together if you like. If you still can't get it to fit, you can cut a few strands off, but do so at your own risk. This practice is generally frowned upon, as it could create a hot spot if you cut too much. As long as you leave 14 gauge worth of conductor, you'll be fine, but again.... not the best practice.
As to your question, you could certainly use seal tight or some rigid conduit as well, though by the time you buy all the parts and fittings for that, you'll probably be nearing your ~$30 mark anyway.
A lot of places have a $50 minimum for free shipping, so sometimes I'll just order $50 of cable, cable glands, and a few other electrical odds and ends, and just keep the leftovers for future projects.