" I just need to find a rubber piece or something along those lines to pt around the edges and complete the look a little."
Try the plumbing aisle in the big box store, look for the 'no-hub' rubber gasket double hoseclamp/corrugated steel(aluminum?) fittings and try to match-up something about the same size or larger, such-that it would fit in the space between the perf. mounting ring & the body piece/fender. You are only going to be using the rubber, obviously, and cut-down. Or, look for a large diameter o-ring. A transmission shop might have something that size, or a truck repair facility.
An alternative would be to go to an upholstery shop and have them use some of their closed cell or solid plastic foam they use to make 'piping' on furniture, and have them 'sleeve' it for you, and then cut wedges in the material outside of where the fabric covers the piping, and contact-cement it in-place.