wonderful idea! But just remember to cut some parchment paper or wax paper to cover the food area. If you have your treats in tart or cupcake wrappers too that would be fine but you don't want your food to touch the painted surface. I'm going to use this idea. I have used many fun pretty vintage object for food display but in the case I'm not sure how food safe they are I always cut a piece of parchment paper to fit and it also makes it look very elegant!
Lovely! Just beautiful the entire thing! I am so going to do this was a wooden cheese board someone gave me. It's cheap but would look great painted with the little glass knobs. I would keep that board. You could use it for all kinds of parties and not just for cake, cupcakes, meat slices, veggie slices, cheese platter, bread tray and so on. Just be sure to cut some parchment or wax paper to size of the board so your food does not touch the spray painted surface and your good to go. You could even change out the ribbon to the seasons or party themes! Lee-ove it! Adorable!
To coat the insides you could take a small amount of a strand of soft yard and dip it in the varnish and run it tough the bead. Just set up a little station at your dinning/craft table. Wear some latex gloves, put news paper down and maybe use a old tray or cookie sheet. Thread several small pieces of yarn through needles and have them ready to go. This way when one piece gets to sticky or messy you have another ready to go.
Also you can sell them on Etsy. Since they are hand made I would tell your story just like you told us. Would make anyone want these special little gems. You might even come up with your own recipes to make custom and special colors. I would start making a recipe box for the techniques you use. Beautiful! Good Work!
For those of you who don't want to use duck tape you should try http://www.mrmcgroovys.com They are awesome and when your done you can pull them out with the remover that comes with them and reuse them on another project! They are made especially for cardboard designs.