I have similar problem. I have a 3 season tent , it seems drafty and doesnt hold heat really well.

I was thinking draping a tarp over it using trekking poles , 550 cord , and small stakes.

Not sure how I will do it . I'll try to work it out tonight.

Mabey I'll post some pics.......................
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Nope, no luck did not work out like I wanted.