Good heavens, don't make one without stabilizer! Just having the stabilizer come loose from some of the shirts I used was hellish. (I think it was from fabric softener build up.) Here's the cheapest price I've found on Pellon 911FF, which is the most frequently suggested stabilizer.
https://www.walmart.com/ip/Pellon-91...Yards/21976551 It's $11.25 for a bolt that's 20" x 15 yds.
The only thing I think about using denim for the shadow would be the difference in fabric weight, and that it might make it a problem when doing the quilting. I had a ball making my T-shirt quilt!