I would think those 4" foam pads that you see kids playing on in the lakes would work great for stopping the ball. A finished piece of plywood would work good also for an approach. If youre just working on fudamentals I would use a older ball that youre not afraid of ruining.

You mean those things the kids float on? That's a decent idea, put something like that in front of a piece of wood.

I have some older balls but none of them really fit correctly. I was planning on using my spare ball though.

For the sliding surface on the approach, I would look at an inexpensive laminate flooring with a plastic/acrylic coating surface for longevity. This can be placed/glued over 1/2 plywood, which is inexpensive and framed with standard framing lumber, as 2x4's, 12" oc lying edge up on the floor.

You can usually find laminate end lots sold at home centers as close outs. Cheap.

To stop your ball, find an old baby crib mattress or two. They are about 5 feet long and some are pretty thick. I've seen them at garage sales.