Matt, I like your thinking on the 3 short (maybe 1/2" tall) rubber feet.
How long do you think they would have to be before someone even notices? I agree, it should pass muster. I did have someone once call me on having 4, 1/4" thick x 1+" dia. rubber pads (stick on). I took them off for a bigger match. Did not want to chance it. Or bring a large mallet to the line and beat the heck out or your shooting position. Then again you may run afoul of some "improving your shooting position" ruling. I have been using a HD moving pad the past 2 years. Works great on grass. It does not slip around like a typical Midway nylon mat. No good in the rain

, so I bring both. Like Danny Biggs, I cut an opening in the
nylon mat for the bag to set on dirt, hopefully grass.