Has anyone tried….Mattress Encasement Cover….

2 lbs… eh… i think if nobody has tried it that would probably be the reason…

That was for a queen – make it a twin and taper it down…probably still at around 12 oz. at best I guess.

ah i see… that's much better.

I have seen a modified version to help keep a ground cover in place. Instead of the cover being just the length of your bag, it is maybe a yard or so longer, then folded up over the bottom third of your bag with the edges sewn together to form a pocket. The end of your bag fits into a pocket. However, because the ground cover is waterproof, you do not want the fabric coming up too far. NB. The tops of bivies are waterproof breathable, not coated nylon.

I've been looking at mattress allergy covers because I have asthma and should avoid dust mites. For mattresses the solution is the various mattress covers. For sleeping bags, you can wash it regularly in 140F water, but I don't think down bags endure too much of that.
So, I've been thinking of making a sleeping bag liner from some mattress encasement cover then washing the liner regularly. There are breathable polyester materials used that are woven so fine that dust mites can't penetrate. These mattress covers are a bit expensive. The cheaper ones don't breath.

I happened to be in a UPS packaging store, formerly Mail Box Etc., They sell for c.$3.00 2.5ml plastic cover for a King sized mattress. This is plastic similar to a platicpainter's drop cloth found at hardware stores.
Waterproof, but not breathable! BTW The thinnest drop cloths make good ground covers.