Go to tractor supply or feed store. Rubber horse stall mat is usually 4 x 6 foot 3/4 or 1 inch thick around $75. Cut to desired size with razor knife. Been using them for years
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Rubber mulch in a box, bucket or barrel works good
Others have said it and it seems your not open to the idea but rubber mulch would be your best bet for the quietest pellet trap. Would take some redesign of your current traps to contain the mulch but if you want quiet it's probably the best and the least maintenance.
I use cardboard boxes stuffed full of old clothes. They work very well for me. Every once in a while I shake out the clothes and remove all the used pellets.
Kenny
I use old clothes stuffed into a shooting box I made. Simple, quiet and effective.