These are some ignition key floats I made for my boat engines. They are modeled after green and red coast guard navigation buoys. They have some internal structure so they print without support but are hollow enough to have plenty enough flotation for an ignition key and the little printable clip.(maybe two or three keys but I would test that)
you can get the print file for the clip here. https://cults3d.com/en/fashion/zortrax-carabiner
I looped the key on a hair tie , then used a bit of monofilament to fish a loop of the tie through the center hole in buoy. Works like a charm and it looks pretty cool too but Chris Thompson cant have any.
.3 mm layers
The model has some minimal internal structure with plenty of void space so you can print solid and it will still float a large key.