Thanks to Angela for this Frugal Baby Tip!
Well, you could always use 100% cotton washcloths, or old towels. If you cut the terry, you'll need to either serge, zig-zag or overlock the edges...basically anything to keep the edges from raveling. Basically, anything that is 100% cotton can work as a doubler (or for that matter as a diaper).
Terry seems the most abosorbant, but flannel works just as nicely. I've used an old pair of flannel PJ's cut up for liners...they were more for poop catching then real absorbancy, but if you zigzaged a couple together or just layed a bunch together you could get a nice quick drying liner.