Just over build it use 1/2" plywood and add more bracing than you think you will need and it should be good to go. I would really suggest bolting the top on rather than strapping you could drill holes and use t-nuts and eye bolts so it is removable and the eye bolts will double as a place to tie down your bicycle. You will want something strong securing the corners might be able to use some padlock hasps for that so it is easily disassembled.