Hello SaperPL,You are free to use any ribbon riser that is ~50 mm long and has straight connection/is not angled. The case is made in a way it holds the riser by the PCI-E slot body and unless there's some weird stuff added to it, it should work okay. If you pick a riser that is fairly standard with the slot being in line with PCB and ribbon, the worst you may have to do is to detach the latch if it's too big, although there should be space for most of the latch types.
You should however look for reviews of specific riser to see if it is actually recommended or whether it's some off-brand no-name company that noone tried to use. We'll be revisiting this topic once we are ready to move forward with
Based on your description this should fit : ?
Straight 50 mm riser long from linkup.