I used a scrap of fabric that I found at the thrift store. It is lined, so I don't have to worry about the rocks wearing against it so much. I put the toys into a bag so I would have an idea of what size fabric I would need (you need to leave enough room for movement). I haven't included measurements because they will vary depending on what materials you decide to use. The plastic is from the packaging for pipe cleaners from Dollarama. The picture of the toys inside is printed on ink jet printable fabric.
I cut the fabric to put in the "window" by measuring the window, then cutting down the middle of where I wanted to place it, folding back the sides, then sewing.
This is the underside after I put both the picture fabric and window in. I went around the outside of each 3 times since my little ones will be playing with this and they aren't always so gentle.
This isn't a great picture, but it shows the bag from the inside after sewing the outside seams. I went around 3 times for this as well just in case.
This is the bag turned right side out, before I filled it.
And here it is all done.