Without specs you can't be wrong

With a big 12 inch your are going to lose constant directivity near 1,000 hz or so listening on axis will be best. if you can accept that, then go with something easy like a 24 inch by 48 inch handi panel. The wide flat baffle with give you some bass support without resonances from an H or U frame. Mount the woofer top inside (mirrored) and enjoy.
Edit: I wanted to add this is going to roll off like a typical midrange so you would need support below 80 hz or so.