If you are worried about heat loss around the cannula, try this do-it-yourself idea.
UPDATE -- Our outside temperature was 18°F. Under the blanket it was 50°F!! We added a foil insulation circle and raised the temperture to 56°F!! Try it yourself.
Winter “Blanket” for Rumen Cannula — Purchase a toilet seat cover. Try to find the old round cover but at least get the smallest one you can find. Be sure the binding material goes all the way around the opening. Get a better quality cover. This one is from Bed, Bath & Beyond. Invert cover (fluffy stuff inside). Cover with plastic bag. Remove air and tie. Keep the bag loose. You could add foam or other insulation inside the plastic bag.
Charlie (the steer) has the #9C cannula which has a larger flange than some of the other 4” cannulae. Put the blanket over the flange and clip around the edges. These are bag clips also from Bed, Bath and Beyond. Continue clipping all around the cannula. Large binder clips might work better if you can find them.
I’m not too sure how much this will help with heat loss, but it is cheap and easy to do so give it a try and let me know what you think.