We don't have the HEKI front hatch cover over our large cabover escape hatch opening but heat and cold were still a problem. It's white plastic cover but they get dirty fast and it gets hot as heck in the summer with the sun beating down on it and cold a heck and sweats in the winter as we often winter camp in temps all the way down to way below zero F. Yes, we do have the Lance snap on winter insulation cover but with it on you can't open the hatch a little to exhaust moisture when wanted or get a little fresh air when desired. My wife goes nuts without some fresh air sometimes as she gets claustrophobia often.
So, I simply cut 2 rectangles about 18 X 27 inches of foil bubble wrap to fit the entire inside of the hatch cover and stuffed it in in a concave shape. Then picked up the one side and used 3M double stick automotive trim tape all across that side, then the next side, and the next until all sides had a total perimeter around the outside edges of the wrap with the double stick tape. Then did the same thing with the second sheet double sticking it to the first. Perimeter sealing stops hot or cold air flowing. Now has double the insulation and never sweats even in way below zero temps at night and totally cool in hot sun and the Lance inside cover is nevr needed on.
Cool in summer sun and warm in bitter cold winters and it looks like it was made that way OEM! Can open it anytime as much as wanted and have true quick escape capabilities not having the snap on inside cover. We want it dark up there anyway for sleeping or napping and NO sun fading. If we want bright light up there or air flow, simple open the cover as desired anytime or in any season. Got a window on each side anyway for light or a little air.
I took it a little further as in -10 temps, the inside about 3" walls around the hatch opening would sweat some and cold drip on you when sleeping isn't pleasant in the least. Therefore, I cut 4 strips of the foil bubble wrap and double stuck them around the entire hatch recess opening walls from molding to molding and tucked the edges flush with the molding so it looks truely professional. Not one drop of sweat since and it's been done on 2 different TC's since 2001 and they have been slept in in -10 degrees and colder many nights in the U.P. of Michigan and up in Ontario Canada in deep winter. It's still up there and still looks like new! The Lance inside cover is under the mattress near the foot end and will stay there.
I remember sending some pics of the foiled hatch to silversand and maybe he still has them.