I made night time doublers for my little one out of an unwanted towel. I just cut to the size I wanted, zig-zagged around the edges of each piece. Put together the number of layers I wanted and then straight stitched (I found this easier than zig-zagging through so many thick layers) Because the towel was very linty, I did encase these terry pads in cotton fabric. They have kept up quite well in the past two years of diapering.
I have also made a few day time (narrower) doublers out of a good quality towel. I was surprised not only by how much pee is absorbed, but they feel almost dry against my daughters skin even when sopping wet. My advice is to use really good towelling if you want really absorbent, soft, durable doublers.
Also, for added stay-dry feel, you could put a layer of microfleece on the top. I think this is helpful with younger babies, but a toddler might get confused by the feeling of no wetness at all (I am talking on the lines on child led toilet training).
Siana.