Question
How to DIY a waterproof LED lamp?
Answer
There are expensive water proof lamps, but if you are a poor hobbyist like me, and have minimum DIY skills, you can buy a cheap, low voltage (DC 5V) low power (0.2W), USB LED lamp (Ref 1 below), and make it "waterproof", using perhaps heat shrink tube or similar goodies.
Or if you are poor and lazy like me, you can just do the following:
References
(1) AliExpress USB LED Lamp Night Lamp