We have heavy latches on nest box and both doors. The slab does give a lot of additional security as well. We also have the 6ft privacy fence surrounding the backyard. Plus we have a american bulldog, a pit mix, and a bichon (scared of the chickens) that have easy access to the backyard. My biggest concern is the bird netting accross the top i do need to reinforce that but between the tarp and plastic thing from kids old playset there has been no issues. The birds seem to love it
Sounds like you gave it some good measures. Are the dogs out at night? Most chicken predators are slinky night critters. Bird netting won't keep out city raccoons or possums. They can collapse their bodies to fit through the smallest cracks.
I love the sleepy content look of your perched birds!