I use a double Greenkids Hemp at the bottom, they are hourglass shaped & are 2 sewn together, I have pulled most of them apart so I can use one or two layers.
In the middle I use a Little Lamb Bamboo Sz2
Then on the top I put a piece of Zorb. This is fantastically absorbant & very thin so adds no bulk.
The zorb like MF sucks up the wetness quickly but doesn't have as good a storage capacity as bamboo/hemp. IT is much better than MF though, so the idea is the Zorb sucks up wetness fast & then it gradually goes through to the hemp/bamboo which stores it so it can't squidge back out.
Puddlekins do nice Zorb boosters which I have had a few of & they are fantastic.
However for cheapness I have also bought sheets of zorb from lushtush and chopped it up into booster shapes, it's like felt so doesn't need hemming, doesn't look pretty but does the job!