Here's what I ended up with for a "home" pasteurizing system for up to 4 gal. It'simply a full-size 6" deep standard steam-table pan (for the milk) set in a full-size spillage pan (for the water) on an electric griddle or stove-top. I know your going "well, duhh..." but I there is one key modification that makes it work better, and that is adding "riser-handles" to raise the "milk" pan an extra 1 1/2" higher than the water pan.
This does three things: (1) it allows more water bath volume (and thus heat capacity) that's needed to heat 4 gal. of milk from 40F to 145F within 1 hr., (2) it provides a gap between pans for adding to and siphoning water from the water pan, eliminating the need for a drain, and (3) provides more comfortable handles for the milk pan. It also provides the advantages and safety of a double-shell heating system and with the siphon you don't need to lift and move the pans.
Here is some more detail (with a video) but the pictures attached summarize it. Note that the grapeseed oil is ONLY for an electric griddle (that isn't entirely flat) to help heat transfer.
Heating data:volume: 4 gal. creamline milk (could do up to 6 gal with 8" deep pan by extending heating time)
starting milk temperature: 45F
pasteurization temperature: 145F
time to heat to 145F: 50 min.
temperature variation at target: +/- 1 deg. F
time to cool to 86F: 20 min.
total pasteurization time: about 2 hr. start to finish
water needed to cool: 4 gal. @ 50 deg.