You can buy the large plastic Easter eggs to use for making these. I would spray the egg with olive oil or Pam before putting the rice krispy mixture inside. Fill one section of the egg with the mixture, make an indention for the m&m's cover with the mixture and fill the other side. Put the two halves together. If the egg has been filled sufficiently the two halves will meld together inside. You would need to quick at filling these or make small batches at a time. We use these eggs for Egg hunts and usually they have candy or money inside. This would be a nice change and great surprise.
And fill with other candy like Easter Reese's Pieces or Jelly Beans
you can also add pipe colored thick royal icing around the eggs to make designs
You can always add a few drops of food coloring to your marshmallow mixture before adding your rice krispies. Makes for a really cute and colorful treat.