Has any one ever used these to mount the bunks on a trailer?
CE Smith Double U-Bolt Bolster and Swivel Bracket Assemblies - Galvanized - 16-1/4"

So I used these to change over my Glastron Carlson CVX 16 trailer from rollers to bunks. You definitely need to use square bolts to mount them to the trailer. It took a while to get them set up properly. I saw online people had them mounted perpendicular to the trailer frame, when I tried to do that they always shifted. I finally mounted them on an angel pointing out and they now seem to work well.

Is there anything better to use?

Maybe something fabricated from a custom shop? I think for their price point they are alright, but definitely nothing special. I thought about having them brackets welded to the trailer, but I guess you cannot weld galvanized to regular steel?

Done all the time.