They are easy to make if you can use a drill and tape measure.
I made some for a friend.
Find or bend some angle aluminum to the size of your choice.
Get the correct dia and length SS carriage bolt and plastic wingnut.
Drill to holes in the Angle aluminum low enough to spin your nuts.
Then mount your rod holder bases to the top of the alum plate.
I have also seen a backer “strip” made out of alum flat bar that was drilled and tapped that was inserted into the inside of the slot then got items bolted to the “strip”. easy way to do it as well, just have to find or cut the “strip” then Drill and Tap.
Maybe not what you are looking to do but it was easy and much cheaper. Just thought Id mention it.