Thanks for all the input. For those that suggested something soft in the the bottom of the back quiver to stick the heads into - I wasn't completely clear in my original post about one of my objectives: some protection for myself and others while fumbling around with loose arrows in the dark.
The Kydex ideas would certainly work, but are more involved than I want to get, and will likely be a little loud banging around in a quiver. Since it doesn't look like the simple plastic slip-ons I remember are readily available, I'll probably just have the local saddle shop sew-up some simple sleeves.