We had planned on using a standard screen door, and reinforcing, but our opening is NOT a standard size, so we had to build our own. Our coop door opens to the outside, so we attached a hook on the outside so it can be hooked open. We mounted the screen door on the inside, so that the screen door opens to the inside. The door itself was made of leftover 1x4's, mitered at the corners and using two L brackets at each corner (I was not going to trust to screws alone). There is one horiztonal board halfway up the door. The door is then covered with 2x4 woven wire, and then with hardware cloth (the 2x4 led to an interesting episode with the cat and the hens).
We open the wood door in the morning and latch the screen door shut, and then close the wood door again at night. I expect we are almost to the point with our NC summer weather where we will leave the door open all night too. We have one small window, and a 6 x 18 screened window in addition, and the pop door, but I think we are going to cut 2 or 3 more 6 x 18 windows this weekend.