While I was in Vancouver a couple of weeks ago, I was surprized to see that they had these cardboard sleeves to hold your hot cup of Timmy's coffee. Here, in the east, if you need insulation, you just ask for another cup.
Already on this trip I had saved a Starbucks sleeve with the intent to see if it fit the Tim cups, but now that I have a Tim sleeve which is slightly a different size, I'll use it instead.
I'll let you know how it works out. If my prototype works, I'll make a template and share with you.