We mocked up a quick external direct fire RIMS. It gravity fed into a keggle on a burner and we pumped it back. I put an inline temp probe coming out of the keggle to watch the temps and just regulated to maintain about 4 gallons in there. It was effective in holding temps but was obviously pretty slow for any ramping.
We doughed in by filling buckets half way with strike water, passed it on to the next team who used a drill driven mixer to stir while grain was added. Pass that on to the next team that dumped it in and passed the bucket back to the HLT.
The HLTs were two 55 gallon barrels that later become boil kettles. The first 50 gallons of runoff are collected in HLT one while HLT 2 is still holding sparge. After HLT 2 is empty, we equalize the two and boil.
I have some video footage of the day that I hope to get up on youtube by next week.