I downloaded a few endcaps, but none fitted my 4040 aluminium profile, so i made my own.
I printed it for my RatRig 750x800 workbench. You can simply change the tolerances in the fusion file if needed. I recommend to compare your profile to the one included in the CAD file first. The fit is snug, but it didn't take much force to fit.
The author marked this model as their own original creation.