how about standing the bed upright against the wall? Or covering it in empty boxes/suitcases etc to make the landing area much higher? Covering it with something waterproof, like a cheap shower curtain, is another option, as long as they aren’t chewers

If they can jump on the bed at 3 months, by 6 months, I can’t see the risers stopping them. Pippi can clear 1 m when he wants, no problem, and can land in the middle of my quite high bed from the middle of the landing, clearing 3-4 m horizontally as well as the bed height. He has been known to hurdle the settee too.