Bon Jour Mon Ami,
Yes, used conveyor belts work very well it just takes time to install them with drilling the holes in the plow to mount the conveyor belt. You also need to have a length of strap iron drilled out to match the holes in the plow to mount them using fine thread hex head bolts, thick fender washers and nylock nuts to mount the lengths of conveyor belt.
If old conveyor belt is unavailable you can always use new round baler belt to do the same job mounting it using strap iron to hold the baler belt in place.
Another option would be slick sheet material from Horn Plastics. They have left over pieces for sale that can be bought in many thicknesses widths and lengths and you can mount them the same way. You will need to cut multiple pieces in longer lengths as a continuous piece will not sag over to act as a snow spray stopper.
The only way that used conveyor belt will work to your advantage is if it is a thin narrow width used conveyor belt as it will have more flex in it to hang down to act as a baffle. New cow stall mats and used thick conveyor belt will not do that for you but if you want a taller moldboard onn your snow plow will work well for you.