I agree, some type of batting is necessary to give that snuggly feel and I've used 80/20 with a nice result. I have only made one t-shirt quilt so I am not an expert but I found the weight of the t-shirt quilt top itself to be measurably heavier. I considered minky backing when making it but then thought it might weigh the entire quilt down a bit too much for the desired use of my quilt. Hopefully, someone with more experience will chime in here. Just wanted to point out that these quilts can become a bit heavy.
Good luck and look forward to seeing yours!