Love your projects and congrats on nice finishes.
For the coaster binding you can cut the backing fabric bigger. I think I added about 1 1/2 inches on each side for the backing. Then fold the edge at 3/4 inch and fold again. You can miter at the corners similar to traditional binding. Press the folded binding and stitch down.
Some directions call for one inch on each side for the backing. I found it wasn't quite enough fabric to get the folds down and look right. This video shows one inch extra on the backing fabric.
https://www.youtube.com/watch?v=npTvVGouO6Y
Enjoy!