I wanted some storage on my 2020 Rad Rover 5 that I could lock and/or remove easily. I added the the following Givi motorcycle trunk on to the standard RadPower rear rack.
https://giviusa.com/collections/monolock-cases/products/b37nta-37ltr-monolock-tech-caseMine is the B37NTA version which is the all black one. The black one with the red reflectors is B37NA. The 37 means 37 liters. They have some larger and smaller ones. I wanted this larger one. Amazon has them, but not on Prime. I got mine at a local motorcycle store called Iron Pony. They do sell online.
https://www.ironpony.com/ecommerce?search_keywords_desktop=B37NTA#/products/view/40995?q={%22keyword%22:%22B37NTA%22}You can use the mounting plate that comes with it. I used that with 4 of these clamps. The clamps let me use the 4 included bolts and put it exactly where I wanted it on the rack.
https://www.homedepot.com/p/Gardner-Bender-1-2-in-Rubber-Insulated-Clamp-2-Pack-PPR-1550/202875615There are a couple of extra pluses and a minuses to this setup. The minus is that unless you're really tall, you probably won't be able to swing your leg over the case and the back of the seat to get on. You'll have to lift your leg up over the front bar in front of the seat (not a problem with the step through Rover 5). On the plus side, depending on the height you have your seat at, the case acts as a short back rest. I think Givi even sells a pad you can install on the case for this. I didn't get that because it looks like you have to drill the case to install it. I love having this storage for all the stuff I like to carry on longer trips(tools, spare parts, food, clothes, etc.).