I'm working on ab shims right now. The way I did it was to layer and glue 2 pieces of ABS. The top pieces equals the length of the shim needed to cover the gap (in my case 5.5"). The lower piece of ABS is 2" longer (1" on each side of the shim). I glued the 1" excess to the kidney plate (since it is 2 layers of ABS, the second layer of ABS lies flush with the actual ab plate. I used ABS paste to fill the crack, and sanded it with 3 different grits. The remaining 1" of excess has a piece of velcro on it, which will attach to the ab plate. I'm in the process of heat bending the shims now for a more fluent fit.
Here's one shim. It needs some touch-up, but you can see how it is perfectly flush with the kidney plate.
Here's the other shim after being glued in, with no ABS paste applied yet.