Floating "Boat Life" keychain. It's a water proof cube with 2mm thick walls so it's durable to handle being dropped, sat on, etc. In the cross section picture you can see the wall thickness and the sloped ceiling walls that help in smooth printing from the bottom up and gradually close the top gap that would otherwise be too wide for a 3D printer to span.
It prints in about an hour and a half. It can hold an average size pad lock from sinking, that's more than most people's keys weigh. So you could be safe to scale the print down in your slicer such as Cura.
If you have suggestions on something different you'd like written on the sides such as the model of your engine or popular boat model I can change the text or put the logo of the brand or image you'd like on the sides and post the STL file in a new design. I can make it raised or recessed text etc. Please only suggest designs that other people could also enjoy printing too.
For added detail you can use model paint and accent the raised text like: