Have picnic ground that is isolated area.
Built a couple with a few 2x4's and some plywood and bought a cheap toilet seat to sit on with hole over a built in bench 

Checked but I guess I have not been excited about them to take many pics.

If you want more detail I could get pics and measurements but I kept it simple with a sheet of plywood front and back, probably all sides and door cut out in front with some framing around edges to keep it stable.

They have been there for many years now as asphalt shingles I used originally have been replaced on 1 and need to be dealt with on second one now

They are not used much as I have never had to clean them out unlike the ones I remember from my childhood around houses and needed shovelled out from time to time

Found one pic after squirrel used 1 for winter storage.  
Squirrel entrance is plugged now.

Hope picture arrives later

(70674221899__60F0DC21-1585-42A0-A604-3456F6BDC616 (full).jpeg)

(70674221899__60F0DC21-1585-42A0-A604-3456F6BDC616 (full).jpeg)

70674221899__60F0DC21-1585-42A0-A604-3456F6BDC616 (full).jpeg (158KB - 20 downloads)

70674221899__60F0DC21-1585-42A0-A604-3456F6BDC616 (full).jpeg (158KB - 5 downloads)