I am in the midst of a similar search. I am not worried about mounting a rear backup site, but about getting a correct center line height for the scope. The iron sights on an AR are about an inch and a half or nearly 40 mm above the rail.
There are several one-piece mounts that are only 18 mm above the rail at the bottom of the rings, for a centerline of 30.7 mm. This is not sufficient for me to get a good clear image even with a very tight cheek weld. Making your search more difficult is the fact that many places do not list the dimensions for their product.
I do believe the PEPR mount will give you a height above the rail of nearly 40 mm even for their 1 inch rings. On the cheap end, UTG makes an extra tall 30 mm ring for which the bottom of the ring is 25 mm above the rail making the center line 40 mm above the rail. However, I do not find that UTG makes a comparable, extra tall 1 inch ring. You could do something with a riser, but it would undoubtedly be more homely and not necessarily much less expensive than the PEPR.