The mk20 is SUPPOSED to already be cold water compatible (don't get me started on this one again).
Do you mean one of the 'oil and rubber boot' type environmental sealing things? If so, Scubapro claim their mk20 doesn't need one.
[FROM THEIR WEBSITE]
"All SCUBAPRO regulator models are equipped with SCUBAPRO's patented T.I.S cold water system, the most resilient cold water system available on today's regulator market.
T.I.S utilises advanced material technology, which eliminates the chance of ice forming in the regulator first stage. Regulator freezing or free flows caused by freezing is primarily a first stage issue as this is the part of the regulator that creates the greatest pressure reduction and subsequently the greatest temperature reduction. This temperature reduction causes the internal components to freeze and ice can form on them restricting the performance of the regulator and creating a free flow.
Most cold water systems are designed to prevent water from entering the regulator by filling them with grease or oil and holding it in place with a rubber boot. This generally requires extra cost and labour charges after the purchase cost of the regulator. T.I.S is very different, it is standard in all SCUBAPRO regulators and it is very simple. Older SCUBAPRO regulators can also be upgraded to include the T.I.S System.
T.I.S has proven to be so effective that all SCUBAPRO regulators fitted with T.I.S comply to EN300, the most stringent European standards for cold-water performance. This is the most rigorous cold water performance test that any regulator can pass."
Hope this helps
