Before the pandemic I purchased a bolt of good quality muslin from Hobby Lobby for this purpose.. Not sure they still offer it that way. It has a nice drape. I forget how many yards was on the bolt but it was well worth it.. Came out to no more than 2 dollars a yard, probably even less.

Do you really need batting? I think quilts without batting is called a Summer Quilt or a Counterpane. I've made quilts without batting. Muslim+fabric+backing is a lot already. Once quilted they are plenty warm for me.

Mother had a box of very thin curtain panels & I’ve been using them for foundations on my charity tops.

I use cotton voile as a foundation and it adds almost no weight to the block. Just a thought for next time.

Watson

I recently made several crumb blocks without any foundation - it worked but not as stable as I'd like it to be. Next time I make crumb or string blocks I will likely try either cotton viole or a super sheer muslin or featherweight interface. While paper probably works well - I do not enjoy the removing process. I appreciate hearing about all the different options and how they work for others.