Nice I also have a Miller rubber keg which I'm using for the MLT. It works great especially with RIMS or HERMS. When I started building my single tier I wanted to keep the prices down but building a nice single tier is far from cheap.. I already have $2600 into my setup.
My suggestion...
You should use nichrome wire..
I am currently working on designing a RIMS tube. I also thought it would be a great idea to use an external heating source. I would wrap nichrome wire around a 1/2" X 12" SS pipe with an elbow on the inlet and a tee on the outlet. Use a 1/2" K type...
I'm building my Brutus hybrid all grain keggle system for $1800. Ordered most my parts from www.HomeBrewStuff.com / www.MidwestSupplies.com / www.modularhose.com / http://www.proflowdynamics.com ... Great stuff, price and service!
If you have the idea of buying some cheap plastic fermentation...
I too have a rubber sided keg which I'm converting to a mash tun. I cut the handles off and added a 12" glass lid. I will be using a heat transfer pipe for the recirculation. I'm waiting on a huge parts order for the false bottom conversion, 3pc ball valves, Loc-Line system for the sparge and...