I haven't finished my sub yet, and I might switch to something smaller with better drivers than I was originally planning to build.
A relatively easy option for Diy is to build a sonotube enclosure.
If you keep the enclosure diameter within a couple inches of the driver, you might be able find mdf speaker rings/baffles that would bridge the gap. It might be difficult to create an air-tight seal using the mdf rings though.
You could also ship one of the pre-cut enclosures + driver deals from parts-express, it might be cheaper than shipping an assembled subwoofer:
They don't include cut-outs for plate amps though, and you would want at least a 15" sealed or very powerful 12" to gain any benefit over the Dayton Sub-1200.
The Bic f12 isn't worth 280$ and the Sw112 isn't worth 450$.
The Dayton Sub 1200 is a well reviewed alternative to the Bic f12, and it can be had for less than 200$ from Solen.ca (after shipping) and possibly less (depending on shipping) from parts-express.com
The Svs Pb-1000 is a step above the Klipsch rw-12d in performance, and it sells for the same price in both the U.s and Canada. It is a very capable sub, and is well worth saving for.
You might be able to get closer to budget with a 15" diy sealed sonotube sub though.