Nice job!

I like the hot melt.  Instant gratification vs. the epoxy resin I used in my switches.

Good job. I used epoxy on one of mine years ago. It held up for ages. But this seems far more secure.

I have moved the subject from Troubleshooting - Engine & Transmission to Troubleshooting - How To Do a Job.

For those that don't have a 3D printer, it's usually possible to get something about the right size (ain't got one to measure!) on ebay. Search "rubber end cap".

I’ve done this job a couple of times using the rubber protective cap from the end of a Stihl combi tool.
https://www.ebay.com/p/1600604733?iid=114385366443
It’s a bit too long but can easily be cut down to the right length. I drill a small hole in the side of the cap near the closed end, unsolder the wires from the back of the switch, insert the wires through the hole, and then resolder the wires to the switch. Next step, where it gets a little messy, I mix up a sizeable dollop of two part epoxy, fill the inside of the cap with a suitable amount, and then force the cap over the end of the switch, pulling the wire cable back out through the hole as the cap is push down over the switch. It’s a tight fit, but it will go down, probably exuding some of the epoxy out around the bottom and through the cable hole - keep a towel or rag handy to deal with this. To finish I apply a small zip tie near the bottom of the  cap to secure it in place while the epoxy cures. 
Installing the two “switch to trans casing” mounting screws is a bit of a tight fit, but they do go in - be careful not to cross thread them.
The author’s 3D printed part is obviously a better way to go, but in a pinch this will work - I just happened to have five combi tools and wasn’t going to miss a couple of caps.

I've done a similar repair but without access to a 3D printer or suitable rubber caps I sealed the electrics with 2 part Epoxy and then wrapped it in electrical self amalgamating tape. It has survived in the irtish damp climate for the past 5 years.

As I am in the throes of restoring one now, I was also looking at how to replace the back cover. As it turns out the plastic caps from several spray cans fit the GPI unit very nicely. A couple of minutes of work with a plastic razor tool will cut a hole. Then either glue, epoxy,to seal it. The red cap in the background was another one that fit and was to be my second choice.