ok he posted and then signed off lol
basically you want a solid foundation for filler/fibreglass. you try the fibreglass and resin and it'll get messy. sooo...
- scuff up the back of the bumper around the exhaust cut out.
- glue/bond a filler spreader or a credit card or anything like that to the bumper so you have a line all across the bottom to work with. stick it with either a bonding material like Sikaflex or maybe glue like aroldite or hot glue gun.
- fill the hole with fibreglass so it comes up level with the bumper and overlaps onto it a bit, rub it down level, then start applying your filler.
doin it as above will ensure it wont crack. if you just try use fibreglass sheets and the crappy resin it will bugger up and more than likely crack and look s**t. using a credit card or filler spreader will give a nice line to work with.