Online at RH you can purchase a single framed skeleton key for $99. I had some keys that I have been hoarding for several years, waiting for inspiration. The shadow boxes were on sale this week at Michaels, I had the paper and spray adhesive from another project. The only other item I purchase for this project was the E-6000 glue. You can pick up skeleton keys from eBay and put together your own for so little. I think I spent less than $20.
I started by tracing the backing onto good quality paper, cutting it out, then gluing it to the front side of the backing piece with spray adhesive. I let it set for an hour.
I drew a blank template matching the size of the backing. I drew centre lines down and across to play with placements on the template. When I found a layout I liked I used the measurements from the template then traced a light pencil line on the backing where I would glue each key in the right spot.
I used clear E-6000 glue to hold the keys in place... Tip! put the glue on the keys, NOT on the paper. The glue is stringy and it will mark the backing paper.
I left them overnight to set, cleaned up the shadow boxes, assembled them and hung them in our entryway.