400W MH, PFO socket and reflector, I just used a couple of metal washers. Maybe a total gap of 1/8", top of wooden hood is warm to the touch but not hot.
I used to have 175W dual MH in a wood canopy. No standoff, just the reflector. The wood is very warm, but not scorching. The stain on the wood didn't even discolor. That setup was used for 3 years until I sold it to my LFS. And he is still using it in his store. It's about 5 years to date.
I use spacers however.. I purchased a used setup and the guy did allot of dyi including his reflectors were dyi.
He placed the reflector directly on a 2x2 strip that ran thru his hood.. I just upgraded the system to HQI and removed the old reflectors to find pure black carbon. Looked like the hood was in a fire.