Cheese wax is nice, pliable, moldable, stays in place, doesn't harden when the weather gets cold. Low melting temperature.
A product used in the scientific community works well, it is called Parafilm. It is a paper-backed, pliable paraffin-based sheeting that comes on a roll. It's stretchy and adheres well to itself. I used to use it to seal the edges of petri dishes. These days I use it when I damage someones ornamental trees with falling limbs from above.
Another completely unknown product that I believe would work is a product commonly referred to as 'ethicone'. It is an adhesive-sealant that comes out of a caulking gun tube. Comes out as a non-slumping silicone-like product, but cures to a fairly hard rubber. There are no solvents or volatile organic compounds in it, thus no odor. It cures in just a few hours with just the humidity in the air. You get this stuff from the roofing supply. It is called M-1 Moisture Cure and comes in severel different colors. It creates and oxygen-impermeable barrier, which, as you might assume, is also impermeable to any other molecule or compound larger than oxygen, which is pretty much anything.
I haven't used this stuff in grafting practices, but the general characteristics of the stuff would seem to lend itself well. Here's a site of the manufacturer,
http://www.chemlinkinc.com/ , but the product is not available through them, just through your local roofing supplier.