I have a homemade grab bar and welded a 3/8” thick tab for the whip mount. It is super solid. I don’t think a thicker tab would hold on yours any better than what you have already do to the thinner wall tubing that it is made out of. Making new grab bars would be an excellent weekend dad project with the kids.
The sides that mount to the frame are 1-1/2” x 3/8” thick flat and the ‘bar’ is 1” tube (unknown wall thickness but I’d guess 1/4 to 3/8”). You may not have the tools or know how but every one starts somewhere. Do you know someone with a welder? Is there a community college nearby that offers a welding class?
My grab bar is bolted to the frame via two bolts on each side. There are nuts welded into the frame.
Another option for you is to just buy a wire feed (Mig) welder and watch some youtube videos. The initial financial sting of the welder will hurt but the long term savings of being able to fab your own stuff will pay for it after a few projects. That is what I did except it was trial and error as it was before youtube existed.