I made this from a $20 waterproof mat from Home Depot, a $7 pitching mound from Amazon, Industrial Velcro and Duct Tape. All in all about $35 total. It is not perfect, but I didn't want to spend $200 on a pitching mat before I knew how serious we were. Super easy!
These are homemade versions of the Tanner Tee. For the "top" part, this fella uses a rubber gasket. I have heard of others going to a local tire shop and asking for an old inner tube...and they would cut the inner tube and use that for their "top" portion. For the bottom stem that is 16" long, I suggest you go with a galvanized steel pipe, rather than PVC.