My earlier post was a bit may have sounded a bit abrupt, and I do apologize about that. I was trying to make the post while logged on from a remote location and was in a bit of a hurry as I typed it out. Let me see if I can expand on that a little bit.
Sealing the edges down. This really only requires a strip about 1/4"-1/2" wide sealed down to hold it while you shrink the covering in the middle. If this is pulling loose then your iron isn't hot enough as you seal the strip down, and that is why it's pulling loose.
Wrinkles in the middle seem to be where a lot of problems, but they aren't that difficult to deal with if done correctly. The first thing you need to know is that you should never EVER put a covering iron on a wrinkle. If you do this the wrinkle will be come permanent and you will NEVER get it out, not matter what you do. When you put the iron on the wrinkle it seals the wrinkled material to itself and won't ever come out. To remove wrinkles you have to work the entire piece of covering, not just the wrinkle. I see people make this mistake all the time, and they often get frustrated with it and give up. Removing wrinkles in Monokote is like removing them from a sheet on your bed (thanks Minnflyer for this great analogy!!

), you can't just smooth out the wrinkles in the middle of the bed, but instead you need to pull them out from the edges of the sheet. To remove wrinkles use your heat gun and start working as far away from the wrinkle that you can. Heat the covering until you see it start to shrink (with Monokote the covering will usually darken a little bit as it starts to shrink), then start working your way towards the wrinkle. As you work it you should see the wrinkle start to pull out before you actually get to the area that is wrinkled. If the wrinkles are in the middle you may have to repeat this process several times from all directions around the wrinkles. But trust me, by doing this you can get your covering tight as a drum and pull all of the wrinkles out. But like I said earlier it takes some patience, because if you get in a hurry you won't get a good job on your covering.
Here are some of the planes that I have built lately, all have been covered using only Monokote.
Hope this helps
Ken