[VAGguy]

I've been eyeballing load stops for that. https://www.yakima.com/load-stop-t-slot
I have lugged a lot of lumber on my rack lately. Full 4x8 sheets of plywood, ~20 2x4s, etc. With ratchet straps, I just hooked the 2 ends together and looped around the end of the bar that is sticking out to keep the load from shifting. Holds up nicely.

 

[VAGguy]

I just measured my wagon OEM crossbars. The slot at the top is 11mm. As best as I can tell the width inside the bar is 20mm. Measured with calipers.

Thinking something like this to slide in with your choice of bolt/eye. https://www.amazon.com/dp/B07L138WPC/ref=sspa_dk_detail_2?psc=1&pd_rd_i=B07L138WPC

 

[VAGguy]

Got these the other day. Nice and solid. https://www.amazon.com/gp/product/B077MNY8NG/ref=ppx_yo_dt_b_asin_title_o01_s00?ie=UTF8&psc=1

Also got some t-nuts, but their base is too thick for the inside of the rail. Trying these out soon:
https://www.amazon.com/uxcell-Stain...:2470955011&rnid=2470954011&rps=1&s=hi&sr=1-4
-OR-
https://www.amazon.com/dp/B077THPQ4...colid=PJ9LIO667KYG&psc=0&ref_=lv_ov_lig_dp_it

You guys, I swear. Making me spend money on all this small stuff. whatever keeps the modding bug going, I guess .