Hi
You can buy thermal covers for the pop tops
Think you'd loose the star viewing
But you'd probaly not need it in the summer nights
Some say a can or 2 of baked beans before bed warms it up up there
Think some have fitted a flexi hose to the over head rear heating vents in the back
And put the flexi hose up through the circular hole to the aft
Then use the bongo heating to heat it up there
But no good in middle of night if your on a campsite and the heat loss would be pretty quick without a thermal cover anyway
Balaclava is another option

Gas safe heating engineer / plumber if you need any advice just shout.