First, the Chelsee is a Sundance I believe so it is full foamed - nothing to worry about there. Any quality full foamed or thermopane tub will be as efficient as you can get. Non quality tubs will be a different story.
Second, don't put anything on top of the cover if you don't lock it - they might become projectiles. If yoiu don't lock the cover, then lock it. If these areas can be got to with the cover latched and the gaps are still there then either the cover is on too loose or you have a problem.
Third, and this has been debated - spa blanket. I notice that without a spa blanket my water is always at set point or 1 º below. Last year with a spa blanket it was always 1º above set point whenever I opened the cover. My take on this is that it doesn't retain the heat as it did with a blanket.
Fourth, keep the bubbles out of the spa. Don't have your spa inject air when you're not using it. Remember that air that's injected (we're not talking ozone bubbles) is at the air temp. My spa only runs the pumps for 30 seconds 2x a day but if you used the pumps for filtering and the jets are adding air - it will cool down the water somewhat.
Finally, remember that the $30,$40 or $50 a month in the winter goes down in the spring, summer and fall - it might cost $2.00 a day in the winter but only $0.50 a day in the summer.
Hope this helps!
