I think your best bet is going to be a separate stand; there are plenty of inexpensive/portable ones on Amazon.

I suppose you could attach a self-adhesive stand like this one, but it would have to be applied directly on the back of the cover, rather than on the folded-back flap, in order to give good support. (Also you don't want to ruin the case's functionality as a self-stand in landscape mode.) But that would mean that the flap has to be left hanging open when using the attached stand.

If you're only needing to use portrait orientation at home, in a stationary spot, you could try propping the mini up against a stack of books or a heavy object. I've stacked marble coasters behind mine sometimes to get a better landscape mode angle for Zoom meetings.