Note: I have had mixed results with how well this works for waterproofing. I keep testing different seals to hopefully improve on this. Currently, I get a little water in when I submerse for 30 min. I think it would be fine for rain but not too much for too long.  I thought I had it figured out when I polished the print but it wasn't enough to pass like Id like. I want to try a softer rubber next.

Updates

The short

I followed this tutorial to minimize voids in the print making it more airtight: 

The long 

Over extruding, small layer heights, and hot temps helped me to get better waterproofing. I have a bucket of prints from all the testing I've been doing. I'm certain the issues I had was from the walls not being water tight.. I thought for sure my setting was good enough but when playing with clear filament I noticed all kinds of voids. So I did a test case in clear PETG and tried to print it as clear as I could.  The result, an hour  with no water ingress. Woot! I used a soft silicone sheet for the gasket but Id like to try the cheaper foam ones again.

If you have idea or experience on this, feel free to reach out (: 

Lots of great metastatic case our there but I wanted one that was waterproof, had a slim form-factor, modular, and could be taken apart. The USB-C port and power switch for the radio are accessible by the side hatch. I also added threads on the back for attachments like belt loops or molle hooks.

Feel free to sell this case as well. I've put it to the public domain for anyone to use as they want. I also added the fusion360 file.

For the battery, I used a 1200mAh 3.7 which lasted for about 6.5days before it reached 20% with minimal radio traffic. 

This case might fit a wisblock GPS but I don't have one for testing. I did add one to the model and planned around it.

I used paperclip wire for the hatch hinge. 

I went through a lot of testing before I could figure out how I could get the gasket to make a decent seal. The trick was to polish the surfaces that made contact with the gasket. This will take out any gaps or micro scratches that will let water in. I used hotglue for the antenna port to make it waterproof but I need to test this still. Both the side cap and the top need to be flat and smooth. I printed mine with PETG. Make sure you test your print before you put your electronics in so you know your case is ready to go.

Side hatch

The side cover gets glued on and a paperclip is used for an hinge for the side lid. 

Parts used

Radio:
RAK4631 Arduino with US915 frequency 
https://store.rakwireless.com/products/rak4631-lpwan-node?variant=37505443856582

WisBlock Mini Base Board
https://store.rakwireless.com/products/wisblock-base-board-rak19003

Antenna for the 915Mhz radio:
ANT-916-CW-HW-SMA
https://www.digikey.com/en/products/detail/te-connectivity-linx/ANT-916-CW-HW-SMA/2694126

SMA cable:
SMA to IPEX MHF1
https://www.digikey.com/en/products/detail/seeed-technology-co-ltd/321990397/15277462

Battery:
1528-1838-ND
https://www.digikey.com/en/products/detail/adafruit-industries-llc/258/5054544

Slide Switch:
MFS101D-14-Z
https://www.digikey.com/en/products/detail/nidec-components-corporation/MFS101D-14-Z/5086543

Gasket:
Neoprene Foam Strip
https://www.amazon.com/dp/B07X4K1QLJ

Threads:
Brass Heat Inserts
Used 3M and 2M Inserts
https://www.amazon.com/dp/B096MD67WB

M2 kit:
I just used 4mm M2s
https://www.amazon.com/dp/B0B6HVS3SJ

M3 kit:
I didn't use this kit but I have one in route for testing.
https://www.amazon.com/dp/B0B6HV8YZL

A little wire

(Optional)
Clear filament for makeshift fiber optic.
Glue for the “fiber optic”