To reinstall the main board, apply hot glue to the front corners of the main board. If you try to apply glue to the inside of the shell, the glue will be pressed in along the tracks and harden up before you can push the main board all the way in.

If you do it quickly enough, you can apply the hot glue and insert the main board before the hot glue hardens up. Otherwise heat up the shell again to soften the hot glue.

Optionally, you can apply either hot glue, or silicone caulk or cement to the inside edges of the lens surround. Silicone is easier since it will not set while you're pressing it in. With hot glue you will need to heat the whole front part of the shell.

You may also use cyanoacrylate on the lens surround since it's more easily available, but it is less waterproof and less flexible.

This still does not waterproof the front assembly- the lens is only friction fit into the lens surround. It may or may not be waterproof, and waterproofing the camera is beyond the scope of this guide.

The purpose of gluing in the lens surround is to cut down on the torque the USB connector board experiences as the battery presses on it from the back.