Craft Foam Horns... I found a DIY page years and years ago with a pattern... but I can't find it anywhere. I'll post something later with the pattern here.
Literally all you need is craft foam sheets, scissors, and hot glue.
You can also do your own version of horns of course, but if you want the headdress mask to be truly lightweight, comfortable, and relatively sturdy, think a while on what to make the horns out of.
I've been very rough on this style of craft foam horns. So far (much to my surprise) nothing's broken irreparably. Just occasionally re-gluing a joint that came loose.
Here's a view from the inside of the mask, all the blue is craft foam
