You could try burying paving stones vertically around the perimeter as a "wall" against digging and "foundation" of sorts. This is what I used for my settup -- "vertical" pavers buried edge-on, and then a layer of "horizontal" pavers on top of those. Frame of coop sits on top of the horizontal pavers, and weight of coop holds it all tight.
Note that one corner did settle a little over time, and our local raccoons worked at pulling out one of the horizontal pavers there. They were only able to move it out a couple inches though, and couldn't have squeezed through even if they got it out. Have since staked that corner (see pic below), and recently added more dirt around perimeter to secure the horizontal pavers in place against those pesky critters.


Note that one corner did settle a little over time, and our local raccoons worked at pulling out one of the horizontal pavers there. They were only able to move it out a couple inches though, and couldn't have squeezed through even if they got it out. Have since staked that corner (see pic below), and recently added more dirt around perimeter to secure the horizontal pavers in place against those pesky critters.
