Over the years, we've accumulated a lot of fitness gear, including two 10kg plates & two 15kg plates, both with a diameter of 50mm. In pre-crossfit life, we would use them for a leg extension machine, (currently for sale) but now they were heading towards being pretty useless. The problem being that the plates have a diameter of 50 mm, while the barbell is 30 mm. That's what you get when you buy accessories 1 at a time.
What to do? After looking online for hours, we honestly could not find anything available that was a proper solution (in the area), one advice was even to buy a 28mm sleeve, which obviously wouldn't fit, duh.
First, saw the pvc pipe through lengthwise, and slide in the piece of pipe insulation. Then pop it on the barbell to make sure it fits properly.
Make sure you wrap enough duct tape on so that the plate fits a bit tight. The insulation material will lose it's size a bit over time. To be safe, I put a collar at the end of the barbell to keep both the sleeve and plate in their place.