I think the easiest way is to lay your bassinet mattress down on the wrong side of your fabric,trace around it then add 2 to 3" all around for a casing.At the edge iron under 1/4 "(i'm assuming you don't have a serger),then fold in again the width of your elastic + a tad more and hem leaving an opening to insert the elastic.1/4 elastic would be the best.

You can make several for what it would cost you to buy just one,and the fabric choices are much better as well.

I made a bunch of regular crib sheets last month for my GD's new baby,cute prints,much better than what you can buy and better fabric as well.
Good Luck
Kathi

Thanks Kathi for your idea. I will send a pic. when I get them made. You are so right about the cost and no one will have any like these. Thanks again, Genny