As you can see in HankB's photo, WSMs can chug along very well even in low temps. BBQGuru makes an insulated cover for the WSMs.
If you have your heart set on insulated, and might do some competitions, definitely find a way to get a look at Backwoods. I went from the WSM to the BWS Party, and absolutely love it.
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18.5" WSM, Large BGE, Backwoods Party
Blue BBQGuru DigiQ DX2
Blue Superfast Thermapen