We need to see a picture of the egg laid in your truck. "If there ain't no picture, it didn't happen"!! But I do believe you.Any lip in the 2-4" height should work to keep bedding from spilling out and leave enough room for them to enter/exit. I think mine are 16x16 footprint, with a 3" lip. One of my hens doesn't even lay in the boxes, she lays in the corner of the coop every day. Parked my truck in the yard a week ago with a window open for a few days and a chicken decided to lay in there, lol.