Ah, I see. the Midwest has a fabric bottom instead of a plastic one. In that case some correx will be useful! Then your liner will sit on top of that.
Liners: Seriously - the guineadad liners are pricy and you can make your own much cheaper. Don't even need to hem or seam. My first go was dead simple:
1. Two rectangles of fleece fabric (the brown with stars pattern in the pics) cut to slightly bigger than size I needed (they'll shrink 1 or 2 inches when washed)
2. One rectangle of Zorb original Fabric cut to same size
3. Make a sandwich of fleece/zorb/fleece
4. Sewing machine round the edge (you could hand sew if you have no machine and are patient!)
5. Trim off a little bit of excess - bit rough at edges but it's fine! This one has been washed a dozen times... mine don't nibble at the edges or anything
View attachment 176968View attachment 176969
6. I got brave and tried to do a 'proper' one by putting 'right sides' together and turning inside out to make a neater edge.
View attachment 176970 but it's no more absorbent and the piggies have no preference!