Good compatible carts for RS Labs A-1 tonearm?

You can use threaded carts with the A1 arm, but the level of 'tightness' of cart body to head shell will be determined by how tightly you can hold the cart to the shell while turning the screw through the two threaded holes. i.e., as soon as the screw head 'bottoms out', that's as tight as you can go.

The alternative is to drill out the threaded cart holes which has the effect of turning the headshell threads into nuts.