I made one last year that was inspired by the Pottery Barn fabric wall hanging. I don't have a photo online, but if you pm me your email address, I'll send you a picture. I don't love it, but it might give you some ideas. It's basically a big rectangle of heavy upholstery fabric with a pocket sewn on for each day. The pockets are lots of shapes and sizes (squares, rectangles, a tree, holly, a stocking, etc.) made of scraps I had. I put the numbers on with glitter glue.
Do you have an Ikea nearby? They used to sell wooden boxes with lots of tiny drawers, maybe 12 each. You could stack them, and paint each drawer. That would give you more room. I found it hard to find treats for each day that weren't candy. I ended up putting a lot of little messages in it, like "Let's get our Christmas tree today!" and "Let's make Christmas cookies!"
For Halloween, I bought a cardboard advent calendar from the Chinaberry catalogue, and my kids like it just as much.
