At work we had lots of parts that were packed with 1/4, 5/15 and 3/8" bolts all 1 foot long. They never got packed with the old parts and shipped back, mostly scrap bin filler. I would take 2 of the bolts and grind the bolt head down. Cut a 1 1/4" pipe and 1 1/2" angle iron the thickness of the bolts. Space the bolts 1/2", weld the pipe to one end and the angle iron to the other. Stand the angle iron end in a vise and use a torch to shape it the way I wanted it.
I made saddles to hold them in place on my boat. A piece of 12 gauge metal with a 5/16" hold in the center. It is long enough to cover both bolts plus a 1/4 to 3/8" hang down on both ends, 1 1/4" wide.
On the back of the boat I raised 2" aluminum angle 15" above the deck. Drilled 1/4" holes where I thought the best location for each rod. I used stainless steel bolts and nuts. Bolt through the bottom, nut on top. Sit the rod holder over the bolt and nut. Sit a saddle on, add a ss washer and a nut.
Kinda like this but without the post going down.
Amazon.com: Monster Rod Holders - 33/45 Degree Fishing Rod Holders Left-Hand-Threaded, 4" Stem, O.G. Black/Red: Sports & Outdoors
I can adjust the holders so rod tips are at water level or straight up.