I’m using an Arkel Bug pannier/backpack that I found cheap on eBay. I have to slide it to the rear of the rack to avoid interference with my shoes but it works great. It clips on tight with aluminum clips so it has worn the paint off of the black painted rack. But it’s never come loose!
I would like a hard sided carrier to fit on top of the rack instead of a bag. I don't know if you have considered this. It would allow me to lock up stuff in the "trunk" instead of an easy-to-get-into bag. I know that it won't provide perfect safety, but it would be a little bit harder to penetrate. I've found that the scooter hard sided carriers with locks have been a little to large for the rack. So I haven't found anything.
I'm using a variety of bags. The adaptation I've made is to install 2 straight pieces of coat hanger wire from the angled post of the rack to the thin bar of the fender. I've put them at slightly different angles and covered both ends on each with black electrical tape.
I don't understand why Magnum leaves such a large gap there or why they make the edging of the rack so thick. Other bike racks cover that area.
Without these wires, my bags touched the spokes. Also, they haven't designed a spot to attach the bottom bag clip as most other bikes have. I'm using the gap, but it's not ideal.
I did try bungee cords ( that's how I got my nickname) to cover the gap, but they ended up coming loose and wrapping around my back gears, which caused my bike to come to a skreeching stop.