It's still pretty cold here in Reno so I'm working hard on my wort warmer. I have a son of a fermentation chiller wired with a cheap thermostat, a Goodwill ceramic heater($5) and a salvaged computer fan. I run the heater through a 10amp SSR(solid state relay) so my heater doesn't melt the thermostat.

I can keep my wort warm enough to ferment in my garage (wife hates the smell of beer) I hope. As soon as that is up and working I'm going to do my first BIAB.
It appears that the best way is to BIAB in an 8.5gal boil pot is to mash in a bag with about 3 gal of water and then sparge with about 3.5gal of water at 170degf. I have two bottling buckets so I think I'll sparge my bag in a bottling bucket for about 20 minutes then slowly drain through the bottling spigot. The grain back should filter just fine.
BTW: I tried just puting the heater in my box but the heat control just didn't work well enough. It was slow to turn on and slow to turn off. ugh.