My openings at 1 3/8". I've tried your (actual) 3/4" with insulation tape and they still clattered when a/c came on. Now I've tried your (actual) 1 3/4" and had to smash the rigid outer band around the filter to jamb it in to fit, but they sound great with no rattle. I think I've bent the grates. The filter material is GREAT tho I haven't found a solution to the depth issue, so I'm not sure how I would rate my overall happiness with this purchase.
Packaging did not hold up to what appears to be ignorant and rough handling by the shipper. There was some damage to the filter itself but with some tweaking was able to install and it seems to be functioning but either stronger packaging or more protection is needed - or a different shipper who takes a bit more care in handling their packages entrusted to them.