Kaymar rear bar - how to lock it closed?

Fred, I'd be wary of using any security that is dependent on bolts that can be unscrewed.
Many thieves are armed with spanners - portable battery-operated stolen power tools such as drills and angle grinders are a gift to thieves - and a lot carry bolt cutters, also usually stolen from job sites.

If it was my project, I'd be welding a 5mm short section of plate vertically at the LH end of the RHS section, that is welded to both the RHS and the flat plate that sits against the bumper.
Then I'd weld a mating 5mm vertical plate to the bumper, at 90 deg to the bumper, next to the first plate.
I'd then drill a 12mm hole through both plates directly above the centreline of the RHS.

This would allow the insertion of a container padlock through the aligned holes and defeat all but the most determined thief.

A container padlock is almost impossible to cut with bolt cutters, and the welding on of the plates ensures that there's a lot of grinding to be done to try and get the locking mechanism off.

The only hassle with my proposal is the plate sticking out from the bumper that poses a personal injury threat to the likes of knees.
However, there are already two plates in the form of the guide plates sticking out, so I guess one more won't make a vast difference - and you can minimise the projection with chamfered edges and rounded corners.

Cheers, Ron.

I drilled a hole right through the C shaped section and the black bar and bought a padlock with a long tang on it and it just slid down and hung underneath.

Cost me about $50 for a good ABUS model but worked for over a year till I sold the whole bar etc.

Never had any problems with cracking or bending at all.

Was a nice easy job and the guy I sold it to thought it was a great idea.

Took all of 10 mins to do the job

There should be a spot behind where the wheel is attached to the vertical bar where you can pad lock it. It's where I pad lock.