If you make your own diaper covers and longies, you can use leftover material or the itchy sweaters you have that you wouldn't want to put on baby.
If you don't make diaper covers, get yourself a wool sweater (thrift stores have cheap or free ones because they have moth eaten holes a lot of the time). Shrink it by putting it in a hot/cold wash cycle and throwing it in the dryer.
Cut a 10"x11" rectangle. If you have a swiffer cloth you can use it as a pattern, but give some extra length to it because future washings may make it shrink up a bit more.