Desktop file organiser or dish drying rack.

You could maybe utilize a heavy bookend?

There are racks you can buy in the kitchen section of many stores designed for this very task. Have you tried any kitchen stores? Bed Bath & Beyond was the obvious choice here in the USA, but they went bust this summer.

Our cupboard has little grooves top and bottom, and we have little wooden dividers that go into the grooves, so we can make as many sections as we want. It was a custom build, but I don't see why it would be hard to add something like that after the fact by taking down some slats with the grooves cut into them.

I see there's also a lot of products like this one.

Some people use spring-loaded curtain rods installed vertically in the cabinet. This creates divided space that the sheets and boards can lean against.

Ikea has the RINNIG or OSTIG plate dish racks They also have the VARIERA pot lid organiser and the UPPDATERA customisable kitchen organiser.

I have some Command strips jewelry knobs that I have attached to the top of my cabinet so when I slide my tall sheets in they catch on the knobs and stand up. The shorter sheets kind of rest on the tall ones.

Vertical mesh filing holders work great if they aren't big. Cost like a couple of bucks at local office supplies.

I installed a wooden strip down the middle for that exact purpose

https://a.co/d/ivMsg40