I figured I would build something like your project, but with a Tesla P4 as I have an extra and that seems to get better results than a 1650. The issue will be cooling the card, but I should be able to duct in a couple 40mm fans to blow air across the heatsink (thingiverse has some ducts that are already designed and ready to be printed and the P4 can be had on eBay for around $90 if you wanted to get one).
Instead of buying a low profile mini gtx 1650 like was used here the faster memory of the Tesla P4 card I have an extra here and it is better as an eGPU.
Comparing Tesla P4 with GTX 1650: technical specs, games and benchmarks.
technical.city
Use two of these 12v battery holders (in parallel).
Then this two pack of 6 Ah batteries with a charger
The same ADT M.2 to x16 riser you used
This M.2 to TH3 adapter
Two 40mm fans
I don't have a laser cutter to cut the aluminum so I figured on just using a project box.
The project box is nearly twice as deep as what you had so I could probably work in a switch to switch between a wall wart plug and the batteries.