Shell to go over a down parka with 13.5oz of down

I think you would want something like this, but made from nylon so it's lighter.

http://www.empirecanvasworks.com/arcticanorak.htm

What size do you normally wear? You could probably get a very oversized windshirt. But if you are already a large or xl it might be hard to find anything big enough.

Stephan…..Have Luke @ Luke's Ultralite sew you up an Argon Wind Shell. The Argon CS90 material is very breathable, will adjust the shell measurements to cover the down parka, you can add adjustable wrist cuffs & pt zips, excellent sewing job, and reasonable in cost with a ~2 week waiting period. Luke communicates in a timely basis….. daily!!

I have used DriDucks in XXL over XL and XXL and it works pretty good. I also use a poncho/tarp.

I can check when I get home. I don't use mine and really never did that much and could sell it if that is the way you want to go.

6.6oz in an XXL form about 2 years ago for the jacket.

NM – didnt read the whole post

Because of the difficulty of covering down parkas, I don’t use my down items until it is so cold that it can’t rain, only snow, but then, I am no expert. I use compact layers like long johns under a thin hoody like the R1, so they fit under an eVent rain coat. A light synthetic like the Atom Lt over the rain coat can work for a while in half-rain half-snow (the most difficult condition I’ve found), then a light but large down parka like a Mont Bell will shed cold snow. A huge wind shell over a down parka never finds its way into my gear list, FWIW.

That's 30% more than some Denali parkas. I can't imagine using that garment in an environment where liquid water exists. Anyway, Rab makes an expedition windsuit for Himalayan and Antarctic climbing to fit over giant down parkas; I believe it's Pertex of some sort.

DriDucks are the worst for resisting sparks. I don't even think they would make a good "sacrificial" shell, as the spark would burn through them so quickly.

You're in dry-cold conditions with this warm parka, so the only reason you really need some water resistance is falling snow melting on you around the fire.

I think one of these would work best:

Empire canvas anorak (as Justin suggested)
Swedish army canvas anorak, or the button-up "snow parka" (same material, but with buttons)
a thin military winter camo smock–whichever one has the most cotton

You could just sew something really loose and sloppy from a cotton sheet. I use the Swedish army anorak, but they're hard or impossible to find now. All of the cotton anoraks are around 2-2.5 lbs I think.

"My aim here is to have a shell on hand while I figure out
how water restiant the shell is."

@ Stephen M:

Your logic seems a bit flawed.

How exactly are you going to figure out how durable/water resistant the outer is?
I guess, by gradually exposing it to the elements in a controlled manner, assisted by your "back-up" shell you are seeking for.
If so, what you need is just that – a back-up piece. It does not have to be a "shell" in the garment sense of the word.
Thus, it could be simply a loose poncho, an oversized Packa, a simple piece of silnylon or PU-coated fabric.
No need to over-think this.

—
edited for typo

I normally take a size Large parka but got an XL eVent parka to fit over my down jacket (see my avatar).

It works very well. BUT my next down jacket, like my new EB Storm Down vest, will have DWR down. That's a no-brainer IMHO.