I remixed the excellent Rugged Box to hold my mobile, temporary installation Meshtastic box.
It currently holds 2x 18650 batteries in parallel and has space for 4 magnets so you can stick it to a steel surface. The plate is modeled for a RAK Wireless 19007 but the plate is seperate so it's easy to exchange if needed. There is enough space to plug in a USB-C for charging if you use a cable witout a bulky connector.
Use some superglue to hold the magnets in place, and if you like, the RAK plate too. I didn't glue mine so I can change it if needed and it stays in place very well just on the friction fit alone.
Update 27-02-2024: the lid is updated with space for some velcro (max 8mm) or a ty-wrap to hold the 18650 in place even if you drop it.
Parts list:
4 of these SbM-209 battery springs:
4 magnets sized 20x4mm
I used this antenna but there are certainly more options:
https://nl.aliexpress.com/item/32811159710.html
The antenna hole is sized at 10mm but that's easy enough to change in the Fusion 360 design.
The author remixed this model.
Version with 18650 battery holder, anntenna hole, magnetic rear and RAK19007 mounting plate.