The simplest way is to use a bath towel, folded into a trapezoid so the narrow end is in front and the wide end in back. It pins on the front sides.
Better is to look at a supplier like adultclothdiaper.com and just buy some flat cloth diapers. Keep in mine that they shrink a lot over the first few washings, so buy them larger than you would expect. Also wash and dry before use, as they come in a state that is not ready for use.
If you have access to a cotton diaper-like fabric, you can simply cut to size, and use as many layers as works for you. Three or four is pretty good.