I have a large property that I service with the ride on but my customer wanted her lawns mulched, not dispersed with the normal mower deck.
My mower deck isn't a mulching deck but I made up an improvised mulching plug using a section of heavy duty conveyor belt. To cut a section of belt to the right size the easiest tool to use is a jigsaw.
The conveyor belt has enough torsional strength to be formed to the contours of the deck sidewalls, completely seals the exit chute and is quickly attached with two bolts and wing nuts. I used two pre-existing holes in the side of the deck as anchor points.
It's not pretty but it works well. Plus, you don't need to weld anything! I started using it three months ago and have had no problems.
Total cost = Nothing.
Tools needed = Jig Saw and Drill.
Time to make. Half an hour tops.