you need to use a fusible INTERFACING on knits/t-shirt quilt blocks....it is a totally different product from fusibles like Heat & Bond which is a 'paper backed fusible' used for applique (it bonds 2 fabrics together) an interfacing adds stability to a fabric- keeps it from stretching, adds body. it is the stuff in collars & cuffs, ties, ect. it is a lightweight lining-with fusible on one side.

Here's more info on fusible interfacing for your quilt:
http://www.quiltingboard.com/main-f1...s-t215946.html

And, yes, the block that you have already done will be just fine after you fuse it to a light weight backing. It will be slightly heavier than the other blocks, but not that noticeable.