I cut off the bag handles and bottoms and ironed the remaining rectangles together to build up a large sheet. (I lost track of how many bags; basically I kept adding one or two until I was happy with the strucural integrity.) I used the iron’s cotton setting and covered the bags with a cotton floursack-style towel I didn’t care about. When I had a thickness I liked, I added some more focal bits cut from other bags to create a bag masterboard. Before ironing:
Then I cut ATC-sized pieces. For the focal points, I cut shapes out of a rejected base experiment where I layered abstract painty elements between pieces of packing tape. (I like that effect as an ATC base, just not for this swap.) A bit of stitching with embroidery floss and they were done.