I just did a build-up using plumber's epoxy. Here's how I did it.
First thing, before removing my plunger, was to take a measurement of its position so I could duplicate how far out the strike plate needed built. Then, I put a layer of clear packing tape on the riser. Before mixing up the epoxy, I tried to visualize what I wanted the rest to look like when complete. Once the epoxy was mixed, I stuck a small chunk onto the tape-covered riser. From there, you have a little over two minutes to get the rough shape the way you like it. The epoxy will harden like a rock within 20-25 minutes so around the 20 minute mark, I pulled the tape and the roughly formed piece off of the riser.
Final shaping of the rest can be done with file, sandpaper, etc. I used a belt sander with good effect. After totally cured and shaped, I colored the gray epoxy with a black felt tip marker and used some silicone sealant to attach it to the riser lining up with the rear plunger hole. A couple strips of velcro finish things off. It took me a grand total of 45 minutes to accomplish. As you can see from the picture, the rest is about 1/4" high, putting the arrow down low close to the hand.
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