I think we all used staples to reattach the cover.  I picked up this staple gun from Lowes:
http://www.lowes.com/lowes/lkn?action=productDetail&productId=77821-355-77821...

I used 1/4" staples.  The staple gun I linked to is just powerful enough.  What I mean is if the head of the staple gun is is placed about perfectly flat (perpendicular) to the pan when stapling and you put some pressure to hold it down good, it drives the staples in nicely.  If stapling an area where the contour of the seat pan makes you have to place the gun at a little bit of an angle and not perfectly flush against it then it doesn't quite have the power.  It does have a power (depth) adjuster that helps some.  I've used it on a couple of different plastic seat pans.  My M50 seat pan had a lot more curves, nooks and crannies in the shape.  It seems like I had to fold and manipulate the cover so that each area that I staples was on a flatter place to get good staple mounting.  YOu can get more powerful stape guns but they'll cost a good bit more.  If anyones used a reasonably priced staple gun that more powerful and forgiving I'm sure they'll chime in here and tell us about it.  

I dont remember what size of staples I used, but you can just take i your old ones to measure with the packs at the hardware store.

thanks