My son loves construction trucks, so I found some silhouette pictures online, converted them into STLs, and then fixed them to make them printable in TinkerCAD. They look fantastic when printed in glow in the dark PLA. I recommend setting the size to whatever you want, then adjusting the z dimension to be somewhere around 2mm to get the best glow effect without it being so heavy that sticky tack won't hold it on the ceiling well.