I am having some trouble finding the right plumbing for the coolant hose that runs into the front of the TPI intake.

I bought this and it doesn't fit:
https://www.ebay.com/itm/New-Pair-10...53.m2749.l2649

It's close (1/2), but slightly too big.

I'd really like one that doesn't rust, but will take anything at this point.

 

If 1/2 doesn't fit then it must be 3/8 NPT. When I had my Miniram, I just bought a close nipple, a 90* fitting and a 3"-5" nipple, all in stainless and attached the heater hose to the long nipple. The close nipple went in the manifold, and then the 90, then the longer nipple. I also left the thread on the end for the hose to grab on the inside and put the hose clamp on. I was able to get all the stainless local, but you can order from McMaster Carr if you want to do it that way. You could probably also eliminate the close nipple and the 90 as separate pieces and get a street 90 to screw into the manifold.