I'm thinking you make a "window" layer, but it would take some time and require hand cutting. Since the bell die itself is only one size you have to make a smaller template yourself. Cut out one bell shape from heavy card stock or grunge board. Trace around it onto your card front if you are very brave, or onto another piece of heavy card stock.
Set the "template" aside. Carefully draw inside the line you just traced to create a smaller bell shaped image. Cut this out with an X-acto knife or scissors. If it is on your card front you now have a window that you can mount the vellum or acetate die cut behind; and add an additional inside layer to the card to cover up the edges.
If you simply cut out a smaller shaped bell image then use that to trace onto your card front and cut the window out that way. Hope this makes sense! Like I said, it's a little time consuming and involves some hand cutting. Not exactly a fast way to get the look you are after. Good luck! Be sure and let us know what you end up doing. I'd love to see a finished card!
Mary in NC