The ones I referred to were square 100 mm box posts.
I would fold two pieces of 3mm plate into u section, stitch them together together to form the socket and stick a square plate on the bottom.
I too prefer square posts, but round ones are easier to wiggle out of the socket
Depends a bit on the agri kit. If it's big kit and a tandem axle trailer it should ride fairly well and as it will have loads of spare capacity, you could load some ballast for the trip to make it ride sweeter and deaden the jolts out.
Looking at the xtreme table, I'm not over smitten with the build structure, bearings running along section for linear rails is one thing. But the feedback regarding support and hassle free use is a big, big selling point!
I've found Soudall seam sealer to be very good. Comes in a silicone gun type tube so is easy to apply, unlike the stuff in a tin. I epoxy primed the body work first, then seam sealed, then colour coated over the lot.
I use Manor coatings Silicone heat resisting enamel on fire pits etc.
Seems pretty good. Only Available in black or silver that will stand proper heat. Iirc other colours only good to 200 degrees C.
Vastly better than the sort of tat that's comes in an aerosol.