HH Supershelter/Undercover and Down Quilt Question

[bwg]

Hi Folks --

I'm thinking about buying the HH supershelter/undercover and I've read here on the forums of folks supplementing the supershelter with various items such as pack towels, KAQs, various thermal blankets, etc.

I was wondering what one may expect if the OCF pad was replaced or supplemented by half of a wide, down sleeping bag or down quilt. I see often on ebay wide, down sleeping bags rated at ~20F and if such a bag were cut lengthwise it would make a decent quilt that perhaps could be fit between the undercover and the hammock.

Is the undercover adjustable and strong enough to support something like this? Would it sag and create air gaps, or would it compress the down?

Would a vapor barrier be needed here?

What practical experience, if any, have you had with such an experiment?