When I was mashing in a 10 gallon insulated plastic Rubbermaid cooler, I built a device to add heat.
I bought a presto pressure cooker with a threaded port on the old through with a pressure dial was present. I removed that, and added some brass fittings, a mini ball valve and a QD. Then I bought some flex copper tubing and bent it into shape. I had the pressure cooker on the burner and the cooler next to it on the driveway pavement. I bent the tubing by hand. On one end was a spiral crimped off in the center, diameter enough to rest at the bottom of my cooler and easily get in and out. I think about 8-10". The other end of tubing then rose upward out of the (empty) mash tun and I bent it into a gentle curve near the QD on the pressure cooker. I attached a QD to the copper tube, and drill small holes (1-2/16" ?? Don't remember). Along the top part of the spiral.
When mashing, I could add steam via the copper manifold and pressure cooker into my mash. There are a few folks I found who do this as well. The only downsides I found to doing this were:
The water in the pressure cooker had a funny smell to it perhaps from machining oil on the brass parts? Give everything a good scrub/soak in PBW first and eliminate this problem
The mash tun made of plastic, eventually separated the inner shell from the insulation, causing my false bottom to be displaced. I couldn't get a seal and had to eventually (after months of frustration before I discovered what the problem was), went with a bazooka screen. Problem solved.
The mash itself was prone to developing a large amount of "teig" which is a grayish colored almost gel like material that forms at the top of the grain bed. Prior to recirculation and sparging, this required a cutting into the top of the grain bed a few inches deep in a tic tac toe type pattern to get good flow. I suppose now I could've simply stirred the whole mess back into the mash and the whole mash itself and be done with it. Perhaps stirring the mash during heating would mitigate this as well. Tis caused no obvious flavor flaws that I could tell.
This contraption was very effective at rapidly heating the mash to whatever I wanted without overshooting temps. I wish I hadn't sold the pressure cooker though, but I have since built a Brutus system and sold off all the old equipment.
TD