I think I’m going to go with 4 dual opposed subwoofers, with the first 2 placed in a slightly staggered fashion just to the inside of the left and right speakers.
The easiest/most effective first shot may be to just place them in all four corners if possible as this is probably less likely to produce peaks/nulls. Better yet, put them at the midpoint against each of the four walls as that is supposedly optimal placement — that’s what I’ve heard from an expert anyway, and it’s free to try of course. FWIW.