If I can't overcome my leaf being perfectly flush, I may be investing in a dedicated router table with an adjustable router plate. When making our showroom bookcases/fireplace, I had some inconsistencies with my face frame because of the slightly tweaked plate. I just don't want to have one more thing/tool that I need to make room for. That's what you'll find with this hobby! There are tools for literally every aspect of woodworking and what guy doesn't like tools???
What is the mounting plate material? I have seen aluminum (tends to be extremely flat, phenolic (comes in totally flat and crowned but I have no idea why anyone would want it to be crowned), masonite or plywood for a low cost alternative. In any case, cutting the opening flush isn't really a big deal- if it's too shallow, it will obviously be a problem, but too deep doesn't matter as long as there's enough material below to support everything.
If it's not flush and the surface IS very flat, drill a hole on the long sides, roughly 2" from the corners and if the material is hard enough to tap the hole, get some set screws that accept an Allen wrench. If the material isn't strong enough to tap and it's more than 1/4" thick, use threaded inserts so the screws won't booger up the hole. The result is a mounting plate that can be made flush and is supported at all four corners. If you want to keep the wood from catching on the edges, bevel them a bit.
Here's Norm, making a router table- if you laminate the top, don't use cardboard strips like he did- it's easy to get small pieces stuck to the cement and the lumps will be there forever. I use PVC pipe because it's clean. PEX or metal conduit would work just as well.
My mechanic told me, "I couldn't repair your brakes, so I made your horn louder."
