I have a Rubbermaid mailbox for sale; it's completely weatherproof as it's made of their signature polyethylene material and has a good solid catch on the door. Has pre-molded holes on bottom to mount to a board/post. It has a red tip-up flag on the side. Outer dimensions are 19.25"L x 7.75"W x 8.5" H. Inner dimensions are 19"L x 7.25"W x 8" H.
From what I can see online right now, this model is selling for $40-$50. It has just been hanging in my garage for about 10 years with a few minor scuffs on it. I will let it go for $25. Go ahead and send me a message here on BisMan if you're interested and thanks for looking.