I coached my oldest sons team when I had a 6 month old. baby At the time I was a single parent.
I found a pack'n'play fold up playpen at a yard sale that worked great. It was big enough he could move around and mesh sides so I could still see him. Since I was moving a lot, the moms would help keep an eye on him as well. My Grandma made a net cover for the top (out of mesh fabric and elastic) so no bugs cold play with him. I think you can buy these pre-made.
The 'soccer crib' stayed in my trunk and I used it for other stuff (park, cook-outs, visiting non-baby friendly homes etc.) I don't remember the brand (it was 7years ago), but they are all similar. Make sure you get one that is fairly easy to wash. The dirt that is not on your baby will be on the fabric of the playpen.