The Radpower rear rack is frequently (and currently) out of stock. It’s a bit hard to find concrete information on what other brands of racks fit on the Radmini, let alone photos of them installed. Some commenters have recommended Topeak racks, but there’s several versions to choose from. I tried the Topeak Explorer.
Here are some photos of the Explorer on a 2019 Radmini. Short version: it fits. There are at least three different holes near the dropouts that the rack can be installed on, resulting in three different heights. The highest hole put the rack closest to the seat but too high above the wheel for my tastes. Using one of the holes that the derailleur guard attaches to (the matching hole on the other side is unused) didn’t work because part of the rack stay rubbed against the bike frame and the rack couldn’t be leveled. I settled on attaching the rear stay to the same hole that the rear fender attaches to. The bolts that came with the Topeak rack are long enough that the rack and the fender can be attached together.
The Radmini’s taillight cable is long enough that, after snipping four zipties that hold it to the frame, it’s possible to attach the taillight to the rack without needing an extension cable.
The Topeak Explorer rack also has a spring-loaded hinged bar on the rack that can hold down a jacket or other light things. It also works with the Topeak MTX QuickTrack system. It A rack without the spring, but otherwise identical, is available for a few dollars less.
So far, the rack and bag (Topeak Trunk Bag MTX EX) are tight and rattle-free.
The Rack: Topeak Explorer (Non-Disc) (W/ Spring)
The Bag: MTX Trunkbag EX.