So I thought it was silly with the Hero7 you had to completely remove the waterproof door to charge inside the skeleton mount so I made up this little splash guard so you can keep everything covered but still get your charger in.
I definitely wouldn't call it waterproof but if a rain storm comes over while you are running a time lapse and have it plugged in, it should keep the rain out with any luck.
I printed in Flexible TPU to get at least a little sealing action and seems to work pretty well but getting dimensionally accurate prints with TPU comes with its own set of challenges.
Hopefully it works for you as well as it seems to be working for me.
I made one for the charging cable that comes in the package and I made another one for the charging cable that comes with most samsung phones.
Its not a perfect replica of the original door but it seems to be quite good enough for the amount of time I wanted to spend on it.
China TPU Flexible