You'd normally put the curve of the bottom of the pickup rings, rather than trying to create a flat area for them to sit on. use some double sided tape to stick some sandpaper to the top of the guitar next to the pickup rout location, and move the ring back and forth on that so the bottom takes on the exact curve of the top. It takes a while but it does a good job in the end.

Most pickup routs for humbuckers have deeper recesses at the end to accommodate the pickup legs and adjustment screws. Some humbuckers have very shallow legs and they aren't needed, but most are reasonably deep (some are very deep), so I'd check to see if you need to rout out a bit of the cavity base for them. Even if your current pickups fit, if you change them in the future, you may need the extra depth at the ends, so it might be best to do it now.

The typical Gibson humbucker rout is 5/8" deep (15.9mm), with 1" deep end pockets (25.4mm), which should accommodate the deepest pickup legs you are ever likely to get.