Having said all that, I may have found a new RS rail that more closely mimics the IUR size & shape more than anything I've found so far. Enter the Troy Industries M-Lok Battlerail!
https://troyind.com/products/m-lok-556-aluminum-9-blk
Regardless of rail, I still have to add some sort of narrow piece of sheet metal to cover the remaining slots from the side rails back to the rear of the rail and add the single picatinny slot at the very rear (as you can see in this pic from Larry Vickers):
