I just had buddy who also put in a Koyorad. He couldn't figure out why he was seeing leaks. Turns out the replacement cap he bought was 20psi. Replaced with a 16 and all seems well now.
No patch material needed beyond the JB Weld. I taped off a boundary so that after it started to harden I could pull off the tape and leave a clean edge. Actually I stacked several layers of tape on top of each other to make an area for the JB weld to pool. It also strengthened the tape so...
I used JB weld once, and it lasted years. In fact, I sold the car with the JB weld patch. It wasn't even Waterweld, just the regular stuff.
Just prep very well with coarse sandpaper, degrease, and make a patch much larger than the hole. Should be fine for a week or two.