From the NRA Conventional Pistol Rules:
"3.9.1 Deflecting Screens Shooters may use a screen fastened to their shooting kit, or similarly placed, to
deflect empty cases, provided the screen is made of material such as "Hardware Cloth," with wires not larger than
1/16 inch in diameter and spaced not closer than 1/4 inch. Screens of solid material may not be used."
Brass catchers for competition cannot have mesh smaller than 1/4 inch also.
Anything goes for practice and beware of old guys that think that you don't want your brass if you don't have a catcher.