5" grinder backer

I picked up a nice pneumatic grinder at a swap meet a couple months ago. Runs well with a lot of torque and has a 7/8" bushing on the nut. Came with a 5" 36 grit pad on it which is all warped. I just picked up some 24 grit pads at HF and want to use a backing plate with them so I can lean on them a bit but control it while grinding welds and such. Seems all the phenolic plates have a step in them which I don't think these pads are going like getting pulled down in the center when the nut is cinched up. I really need a flat plate, or am I going about this wrong.
Grinder is a commercial version of this
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These are the pads I plan to use. They don't have adhesive or velcro, nor do I want them for this use.
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and the backers that I can find.
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