Patience Grasshopper.......
Joking apart Kevin,it can be frustrating.
Try and fit the rubber to the cab aperature and if necessary use some masking tape to hold it,as I mention in my guide try and 'cram' the rubber as you go to ensure it's good and tight on the pinchweld.
Cut your ends as square as you can and allow an 1/8th to 1/4" oversize to allow for shrinkage and apply a small dab of rubber adhesive or superglue at a push.
I use urethane as it's what I've usually got to hand.
