Fiberglass and bondo aren't a good idea for this type of thing because bumpers flex and they don't. SEM makes adhesives to bond and repair these bumpers and also a woven mat to reinforce the joint or repair. There are also welders that join or repair plastic bumpers. There are staplers that create mechanical joints. They melt a staple into the plastic and leave the legs sticking out to be cut off. I've been out of body repair for a while, so there may be more and different systems out there. Check around with local body shops or at your community college body shop. Most shops will only be slapping on new bumpers but some will be repairing bumpers not available on the market. Remember, as rigid as steel is, after a while, both fiberglass and bondo cracks.
Jack Byrd