Down crewneck "sweater" or "sweatshirt"?

I think you would be better served with a pullover with a zipper or button neck.  A down garment doesn’t stretch so if it’s loose enough to put it on with a crew neck it probably won’t seal as well. Just a guess.

Is there a button neck you’d recommend?

If you’re looking for something without a collar for layering purposes, I’ve been intrigued by this from Montbell, but I don’t have any personal experience with it:

Superior Down Round Neck Jacket

There’s also this and this, for low collar jackets with full zip at a low price.

I seemed to remember Backcountry.com carrying a house brand down pullover with neck snaps that met your requirements and came in at a low weight, but I didn’t see it on their website today.  You may be able to find them in Gear Swap.

Patagonia offers this one.  If you’re not looking to break the bank, I’ve been really happy with my 600 fill down garments from a cost per ounce perspective.

https://www.backcountry.com/patagonia-down-snap-t-pullover-mens?skid=PAT01JS-ASHTN-M&ti=UExQIENhdDpNZW4ncyBEb3duIEphY2tldHMgJiBDb2F0czoxOjM6YmNzQ2F0MTExMDAxNQ==#

Keep an eye on Eddie Bauer as I’ve scored some great <$100 ($30 in one case) deals on down gear over the years, but I don’t see anything on their website at the moment that meets your exact requirements.

There’s also this and this, for low collar jackets with full zip at a low price.

Actually, now that you mention it, I’ve got something almost identical to the second one you linked that’s packed away with my hunting gear that I forgot about.  I haven’t used it for backpacking because I try to keep it from picking up any scent, but it is light, warm and inexpensive.  It’s kind of delicate, but it’s a good choice for layering

Thanks all, interesting!