I've been looking at the set below from Amazon.  I haven't tried them yet as I am still awaiting delivery of my bike. 

It looks to me like you would be able to cut/drill holes in the fabric top for the bolts to pass through so that you could mount it between the seat and rack.

https://amzn.to/3qHf5IZ

Has anyone tried this Panniers Set from Amazon?  You'd probably need to cut holes for the bolts, but other than that, it looks like it should mount easily enough.

I'd probably go with the black  but linked the gray as it is easier to see the details of the bag.

https://amzn.to/3quAZyQ