The tank doesn't need to be stainless....just NOT carbon steel.
I've used aluminum, a ceramic crock-pot for small parts, and once even 4" PVC pipe....the PVC needs to be well supported, though--as it heats it will warp and bend.
For $5-6 you can get enough solution to easily do a few long guns...if you're crafty you can find a *tank* to use for pretty cheap.
If you have resturant supply houses nearby, look for steam-trays (like you find at most buffet style resturants). You can usually get them up to 24" in length (rarely longer, but possible) and they work great. If you know someone who welds stainless, but two and hack 'em together. They're pretty cheap ($10 or so each).
I always parkerize before duracoating to promote adhesion. I don't care how even/perfect the park job looks as it'll be 100% covered...but I've only had one item come out splotchy--the rest have been pretty sweet!