Was thinking of creative ways to modify it and make it waterproof.
You're not looking for waterproof (can work when placed underwater), you're looking for weatherproof (can work when showered with water.)
Silicone or soft-rubber gaskets will also do the trick. You may have to make your own, given that most are for NEMA 5-15.
https://www.amazon.com/SOCKiTBOX-We...ck+Weatherproof+Gaskets&qid=1577823432&sr=8-7
If making a box, try to design it so that the cables coming out are going downwards, not lateral. The idea is to engineer the drip line so that water dripping down a cable cannot go into a box. Ensure that the cable coming out of the car's charging has a low point lower than where it enters your box; the water won't drip upwards.
Pre-made box designed for proper drip lines:
https://www.amazon.com/SOCKiTBOX-We...ck+Weatherproof+Gaskets&qid=1577823432&sr=8-7
Notice that the lid locks the cables into downward angles.
Lastly, if you're going to plug your mobile charger into a cable via an adapter, it's much better to use the right NEMA adapter right at the Tesla end:
The last two are 14-30 and 14-50. If you connect the Tesla 14-30 adapter to the mobile charger, the car "knows" what amperage it can safely draw, and will try to charge at 22 miles/hour... but if you connect the Tesla 14-50 adapter to the mobile charger, it thinks it can draw more power - 30 miles/hour - regardless what adapters/wiring you put in between it and the outlet. You'll need to remember to manually limit the draw from the car's screen or else you can blow the circuit or cause a problem in your cabling (melting, burning fire.) And you need to remove the manual limit when charging somewhere with the actual power rating.
Summarizing:
A) A Tesla 14-50 end connected to a 14-30 adapter plugged into a 14-30 wall outlet
can (safely) deliver the same power as
B) A Tesla 14-30 adapter plugged into a 14-30 wall outlet.
The difference is that with B), the Tesla knows what it's really connecting to and is smart enough not to do the wrong thing. With A), you'll need to makes sure the car is always set properly.