Mark, for larger-than-stock forstner bit sizes of holes, of any shape, I make a female template, basically a hole cut in a 1/2" plywood plate with a jigsaw and filed/sanded to final size. Yes, you can file wood! With your router fitted with a stock collar, it's a cinch to cut out any shape accurately, the collar follows the template. I use a 30mm OEM collar on my Makita and usually 1/2" [12.7mm] diameter straight bits which means a 2" x 3 1.4" hole needs a 17.3mm bigger hole; say nominal 99.8mm x 68.1mm. I use electronic vernier calipers to assess template sizes. The radius left in the corners can he filed out with a sharp rasp. You need to fix the plate to the surface to datum lines with clamps or double sided tape, and you need clearance for the router base of course, [ not always possible, that's when you decline the work!]. However, the method means it's impossible to cut the hole too big. Carbide tip cutters will cut practically any material except nails. I use this method to house newel posts, handrail sections, cut mortises, etc., and easily get accuracy of less than 0.005" When someone of limited skills and intelligence is butchering the job and delaying you getting paid, I reckon it's time to do your own.
Alan
