To correct the condition a piece of steel or aluminum would have to transverse that opening since the problem is in the semi-circular piece (behind the opening) that connects the two fences which are not in alignment with each other.

The shims under the ends of the fence and a piece of Bakelite, or something similar, between the clamp and fence should work.

I'm sure I was "preaching to the choir" with the heatsink.

I'd go with kickintile's suggestion of using a TS table and wet/dry paper as a lapping surface and simply sand the fence to true. This is very similar to how I dress waterstones for sharpening knives when they wear hollow. Some careful work at the end will ensure things end up flat and square.

I've found using wet/dry SiC paper with water, it will adhere to the ts table without tape or adhesives, just capillary action from the water.

J