I have been doing this for a few winters now I put a screen over the hopper grate because when the fin fish swims down stream...lol and lands in my hopper I hate having to try and net it....lol But seriously the screen had saved me many times from possible plugs and this year my boss welded 5mil rebar spaced 2 inches apart and it works perfect I have seen so many pieces of precast that have remained on the grate and not gone down in the hopper.

Some ready mix drivers are good at catching it but most have a hard enough time trying use that one finger to discharge the hopper let alone multi task...lol The only thing with the grate or a screen you have to run the vibrator all the time and the rebar grate is good because you can just lift it off where as the screen or chain link fence has to be attached with zip ties

I like it

I like it also there are all kinds of objects that fall out of a mixer, have you got a picture of the rebar one.

 I gave it a try but it the low slump sticky mud dont go through it very well, even with the vibrator going.

 I finally found me a couple pieces of 4" wire mesh, you can slide'em over each other and adjust the grid size.