One tip that helps avoid failure when you're working an open area near a corner...............

Iron down the covering over wood with some pressure. Then let the piece sit for awhile. That area needs to cool before the glue is subjected to tension.

After a sufficient time to cool, use an iron only over the open area. Don't get heat into the solid area while trying to shrink the open area.

The iron's heat can be controlled closely with the dial control. Keep trying with a higher and higher setting. It is often better to use the square, rear of the iron in stead of the pointed front, to get into a corner.

On a recent ARF covered in Monokote by professionals (the Chinese who mfg'd it do it for a living, right) almost every corner that could have a wrinkle like you picture had at least one. Even with my technique, I couldn't get some completely out. Since then, I've passed over buying a couple of ARFs I'd like to have simply because I couldn't see betting the MonoKote was going to be any better on them. Until I want one bad enough to figure recovering into the cost in time and money, I guess I'm just going to keep passing.

FWIW, I've collected a number of the little electric WWI biplanes Tower sells. The 1st had some areas that absolutely would not tighten up. The 2nd was worse. The 3rd was slightly better but the underside wouldn't tighten. It looked like they had lighter monokote on them but knowing the reason they look bad doesn't make them look better. Recently I decided to take another chance and got the Nieuport. It's still in the box and until I have nothing better to work on, and it's raining, and nothing is on TV (which I seldom watch anyway), I might take the time to clean up the shop. Then I might stick it together. Or not.