Part of the issue it tools. When I made the last quarter panel antenna mount, I purchased a sheet of 1/4 inch think aluminum from http://www.speedymetals.com/
As I recall, it cost just ≈$40 including the freight.
I cut the pieces I needed to fashion the mount on a bandsaw, with a metal cutting blade. I curved the fender contoured piece in a sand pit, using a rubber hammer, and a contour duplicating tool. It is like this one: http://www.amazon.com/General-Tools-6-Inch-Contour-Duplication/dp/B00004T7RA
It took about 4 hours to get it right!
The metal shop heliarced the pieces together, which cost ≈$25. The hardest part there, was to make sure once the mount was bolted on, the flat surface for the antenna base would be parallel with the ground. Whew! I had a friend hull blast the surfaces clean. I then had a local body shop powder coat the mount to match the color of the vehicle. That along was ≈$30
I attached the mount with 6 stainless steel bolts, right through the left quarter panel. On the inside was a second curved piece, which spread out the stresses on the quarter panel. All told, it took me about 4 weeks from start to finish, and about $100.
Yes, it is a lot of work, but when you want to mount a near 20 pound antenna on a quarter panel, it takes time and patients! This is one area of mobile operation, that it does not pay to get into a hurry!