For that flag, I’d use flannel. Wash, dry and iron it before you use it. It’s inexpensive, and you can buy what you need. (Although, please buy 1/3yd exrea for the shrinkage.

I suggest felt. Yes, felt that is on the bolt, not the small squares for craft items. I believe the felt has some polyester in it to make it stronger. I have used this for table runners and wall hangings--I wash it first, It is very wide, has no grain, and comes in many colors. It is easy to work with and gives a nice result.