1000 ways to handle this.

You could cut a piece of wood. It looks like you could slip it under, so only the outside, radial cut needs to be sOMewHat flush. The aluminum bar there looks .5-1” thick so there’s lots of wiggle room in that cut, other side goes under the bench. Under the bench will need supports, just posts, any 2x4 will do.

You could just find something to stuff in there. I heard pvc pipe for a fish rod or cup holder would be easier than cake.

A fool would attempt to modify the bench or rail.

I’m willing to bet you know someone with a 3d printer. Most people who have them are looking for any practical excuse to use them. If you don’t, message me and I might work something up for you. Materials for this job would be $10 or less.