Look at this link. Just the rod holder part.
Attwood Clamp-On Rod Holder - Walmart.com
I made mine kinda like this. For the loop I used a 1 1/2" pipe cut in sections 3/8" thick. For the fork U used 1/4"x 1 1/2" angle iron cut 3/8" thick. For the center I used 3/8" rod cut at 12" spaced at 1/2". I made some saddles out of sheet metal so the rods won't spread. They sit up a few inches on 3/8" thread all.
I made these in '02 and '03 before I bought my boat. I've changed up the mounts some but still use the holders.