This is my ugly prototype but here’s mine with the same battery and a psychotiller custom. It’s never fallen off and the magnets make it easy to put back on while it’s on the ground.
For the fuel guage I cut a lexan window and attached it with sugru around the edges. I beveled the window with sand paper.
@psychotiller threw in some small strong magnets with my order and I used a dab of sugru to glue them inside the enclosure around the access holes. Then I placed magnets on top, stuck to their counterparts. Each with another drop of sugru on top. Then I squashed a piece of lexan onto the magnets at let it dry.
After that was dry I put a layer of sugru on the lexan between the enclosure, careful to leave the magnets exposed and as close as possible for a strong hold. I also stuck a little sugru ball on the lexan to fill in the acess holes. Before fitting it in place I covered the enclosure with soapy water so the sugru wouldn’t stick. Then pressed it into place and molded the edges into a seal. Then squished the little sugru lumps with my finger from the underside to fill in the seal and lift the whole thing up off the enclosure for drying. I went back when it was dry and covered the top and added a little tab for pulling it off.
It’s rough and ugly. Someday I’ll make a nicer one that’s not so lumpy but it works great!
I told myself I’d cut a hole for the usb ports if I ever experience a situation where I need to charge a device with my board. So far I haven’t felt a need to use it.