Would just plain muslin work? I'm hoping for something as inexpensive as possible....

Definitely!

Sure it would. No one is going to see it once it's up on the wall.

Yup. Anything goes for a backing on a wall hanging. I even found some pieces of ugly pre-quilted material and am in the process of using up that hideous bit on a UFO. No need to add additional batting, as it is stiff enough as is.

Oh good - thanks! I was afraid maybe it wasn't good, that it would sag or something. :)

anything works because no one will see the back of it.

I almost always use muslin unless it's a gift for somebody else. On the special keepsake wall hangings I made for my sister in laws I used a large scale print that complimented the front.

That's what I use. I also use it when I'm making a practice block. I finish it then put it up on the wall.

Dingle, I LOVE your boxer!!! :D

I almost always use muslin for wall hangings. Cheap muslin, at that.