I'm gathering materials to build one right now......Got any build details??
Treated 4"x4" sill, 2"x4" frame and rafters. Random width 1by whitepine rough sawn boards applied vertical. Should have ship-lapped them but was in a hurry. Later applied lath on the inside to seal vertical cracks.
Treated 2"x4" floor joist. Rough sawn white pine boards for floor...do not nail, rather screw them down for easy removable when the time comes to empty hole.
Treated 2"x4" frame for seat bench. Front of bench white pine boards and top of the bench planned fir boards (don't want any slivers). Don't nail the bench boards, screw them in so you can remove them easily.
Roof sheeting is 3/4" plywood, covered with roofing paper. Aluminum roof edge and rolled roofing.
Door is 2"x4" Z frame and covered with 1"x8" vertical car siding. Strap hinges and latch is barn door type.
Screw in two guy anchors opposite diagonal corners. Attach to side of building with pipe U clamps.
Ventilation. Left opening above door and underneath of shed roof and screened it in. Hole in bench is sufficient lower vent. Do not make a cover for the hole because condensation will form underneath it and when really cold the cover will freeze shut.
One last thing, very important. Cover the inside of the vertical boards on the front of the bench with a piece of linoleum flooring. You don't want to be peeing a stream on the boards. Don't make the mistake I did and apply aluminum flashing. The urine ate it up in no time.
Pic of inside.