Wanted to post an update.
I built the 3/8" copper HERMS coil, for a total of about $40. Took about 30 minutes to solder together and clean up.
I used it last night and for the first time maintained a constant mash temperature for 60 minutes!
This is very exciting, and I'm really wondering why I didn't do this sooner. it was only $40 in material.
Note:
I did not recirculate the hot water in my HLT around the coil in any way, so I'm assuming if I had have, I would have transferred heat more efficiently through the coil. But to be honest, I don't think I'm going to bother, as this requires an additional pump and further complexity to the recirculation. To compensate for not recirculating the hot water, I had to set the HLT temp a little higher to get the desired heat transfer.
To maintain a mash temp of 152F, my HLT was set to 170F. If I recirculated the hot water around the coil, I probably could have set the HLT tem to something like 160F....and if I made the coil longer, probably even lower a temp....but like I said, to avoid further complexity, I'm just going to set it to a higher temp for the same results.