Yes my 15 trailer has one like you describe.
It has a flat base plate with turned up ends (to stop it sliding out from the U bolts and special U bolts (i.e. not U but right angled - bolting to flat plate not round tube).
What worried me last year is that one of the turned up ends on the bottom plate broke off ! Must get round to welding it back on.
My 18 trailer has one that is welded to the main front trailer beam and also has a 2 foot angle bar from the main bar to the support to triangulate the load. I use that with a special bracket on the back of the box to bring all three masts home in the winter and it does sway about a bit
If you get no joy with autow Towsure might be able to help but they don'seem to do much marine stuff otherwise I know it is quite a round trip but we are doing some welding on and off at the club at present.
My MIG welder is probably a bit light for a deep weld on the trailer but Chris Stafford and Dave Lincoln do arc welding so if you are interested I could ask them - send me a photo of the problem piece if you can get one to ssc at ehvpa dot com
Steve