I was the lucky recipient of a free hot tub. Unfortunately, it was leaking profusely from both ends of the heater element tube. The culprits were something like these: https://www.poolpartsonline.com/p-103513-replacement-snap-nut-black.aspx

I also saw the split nut at http://www.aqua-man.com/row_num.asp?Ic=42894 as an alternative design. It seemed to be an easier design to copy (and a lot faster than ordering it for $15 off Amazon).

I designed everything in OpenSCAD and printed in ABS prior to a wipe down with ABS slurry and assembly with 1/4"-20 hardware. You will still need a washer in between the heater element and the PVC pipe the nut is screwing on to. I just reused the silicone washers that were previously there.

The design is parametric so you can modify it for other hardware or for other heaters/pipes (though I believe spa/hot tub heaters are fairly standardized on this size). Unfortunately, you can't use the online customizer because I am using a thread library (see source object for link to it; I included a copy so you can just unzip the file download and immediately open it in OpenSCAD).