One of the bits I was reading on the HMF claimed that most of the bacterial colonization is going to be in the first inch, and any thickness over 1" is less for surface area, and more for structural reasons.
Assuming that's true, the minimum thickness of the HMF is probably proportional to lenght/height, and possibly shape (I would assume an arch/curved HMF would be stronger then a flat panel...)
I've got an quasi-corner in one of my tanks now, been running for a couple months. Mostly did it to hide hardware and keep critters out of the filter intake. Snails seem to like it.
I've also got another tank I'm hoping to get set up in the next month, it's an 8gallon hex that I stuck a 1" mattenfilter curving out from the back panel. Again, mostly to hide the heater. I haven't tried it with the pump running, but since it's a pretty narrow piece (maybe 8-10"?), it seems fairly stable with only a 1" thickness.