Thanks to Laura for this Frugal Baby Tip!
Do you have some of the Kooshies flushable liners? They are a rectangular paper liner which you can just flush down the toilet. I don't know what the exact measurements are, but I like to use them because they are a bit bigger than normal. Use this kind of liner to get your pattern, or do it like this:
Make the liner about half an inch longer than your diaper, and about an inch wider than the crotch part. It doesn't matter if some of the fleece is sticking out when you've done the diaper, it doesn't wick. And you can be sure of catching all the poop if you make sure the liner is a bit bigger than the diaper. Very dry and comfy for the baby too.