Hi. "It seams" like a 6.5 square with a 1 inch seam yields a 4.5" block lol assuming 1inch on all four sides. Either make your square 7.5 or use a 1/2" seam. Personally I prefer and use only the one inch seam. It curls up nicely and is easier to clip. Denim and flannel will be plenty heavy w/o the middle layer. One thing about denim is it comes in weights. I never thought abouT it til I made a denim qayg and noticed the weights on the bolt labels. I used a 6 oz. for my qayg an it was plenty heavy, but if I was making a garment I might go with something a little sturdier such as a 10 oz, which does cost a bit more. Have fun.
Leave it to me to do my math wrong the first two times. This is why I have my son double check all my math when I map it out. Silly me, 6.5" would yield a 4.5" block. I have a 6.5 ruler, but I might just have to make them 7.5. I will know more about what I can do when I get the seams out of all the jeans. It's going to depend on how wide the legs and stuff are. The best man at our wedding, 13 years ago, passed away due to cancer. I'm making his brother a memorial quilt as all Mike wore, even to our wedding, was big overalls. Everyone called him Big Mike. He was 6'7 and a good 350 pounds or more back before any of us thought it was cancer. I saw a pattern for a rag quilt with denim and flannel and it alternates every other block. That's exactly the stuff he wore, so I know I would honor him well with that pattern. It looks like everything he ever wore was bibs and flannel. His shirts were still in good condition, so when he realized the cancer had gone too far and he needed to get his affairs in order, he donated most of them to Goodwill himself. I've been all over town and I can't find them. I'm going to just buy several yards of plaid flannel as I think the flannel is the only fabric I can think of that I can pair up with denim. A regular cotton would be two thin.