Those are Way Overpriced!
You may need to do a small mods on your gunnels, weld a plate or tube with a welded gusset brace. If you are in tough river water, I'd suggest you weld a short length of bare frame tube on top of your gunnel, you can slit it longways and get some support and mounting points. It's good to use the clamp-ons because you need to be able to adjust until you find the sweet spot that fits the row pivot point and correct proximity to your seating.
I'll look for one on this site, think it's posted in "fly fishing jon boat" posts with pictures. The member has a bunch of pics on instagram of his set up 1648 and guides south of Austin TX.
I use these: https://www.nrs.com/product/91040.02/nrs-frame-6-oar-mount
https://shop.clavey.com/steel-hd-oarlock-stand---bolt-on-p1332.aspx
https://downriverequip.com/shop-products/oar-tower-rainbow-bases-w-pins-pair-/
https://www.ebay.com/i/253689565402?chn=ps