"I've used screen for 33 years now. I've used the limp stuff, the super stiff Black stuff, and probably 10 types in between. I've settled on a grade of STEEL screen that I would probably describe as an '8' on a scale of 1 to 10 as to it's 'stiffness'. On a PROPERLY adjusted trap, a screen can be a huge asset. On the other hand, and I've experienced this first hand 1000's of times myself, screens can be a detriment. The key is having your trap pan level adjusted correctly. It seems to be an issue with people, and I deal with it all the time. (helping people adjust their traps). It pains me to see and hear that people try to use screens with the pans of any trap adjusted so the pans are ABOVE jaw level. With the super stiff screens that I see people trying to use, no wonder they have so many pre-maturely snapped traps! If the screen can't sit flat on the pan, there is no way to keep a 'hump' from forming over the pan, that is just simple logic. AS for 'forming' the screen over the pan, I put that in the same category as 'loading' your snares. Are you kidding me? You should be able to set the trap, raise the 'free' jaw, and slide the screen under the jaws. If you learn to 'cup' the screen a little,(in your hand) it will slide under the jaw corners easily. The screen will simply re-gain it's original shape. If the trap is adjusted properly, the screen will be flat.This whole process should take 5 seconds. If there is a hump, especially when using the super stiff stuff in vogue right now, you very well might have problems. You are asking a lot of a super stiff screen when you want it to increase 'kill area', and THEN to let the screen flex enough to take up any and all slack that you have created by not cutting it to the right size, or leaving/allowing a hump to form. I use and sell pre-cut screens for the #3 Montana's. They are 5 1/4" W X 6 1/2" L. They are made of the material that I have settled on after using them for years. I'm using these every single day, and I know what works for ME. And, I also talk to and help 100's of people through PM's and e-mails, and phone calls. You don't have to make things like this harder than they need to be!"
This is probably the best write up I've seen regarding screen to explain it written by John Graham on here.