Some people try to get a large spoon or ladle under them to catch it. I've never had any luck as Tess sits down and faces out, "No messin' w/ mah butt!"
Others replace the litter temporarily w/ aquarium gravel.
Another way is to put a layer of plastic over the litter .
If you need to check frequently because the cat is prone to ketones, I'd get a TidyCat Breeze system box. The "litter" is non-absorbent pellets and the pee goes through to a pad underneath. You just take out or cover the pad to catch the sample. BTW those pads really decrease the smell!