>>2924185
look how they do pay boxes at bumfuck egypt campgrounds. theres probably a usda forest service build spec sheet out there for the dimensions. its like a piece of 8" x 3/8 wall square tube with a door welded inside. being youre asking here you probably arent equipped to build it though
here i googled it for you
https://www.fs.usda.gov/t-d/pubs/pdfpubs/pdf98232343/pdf98232343.pdf
another
https://ironrangersllc.com/indexpage2.htm