This is an idea I’ve been playing with for a while now.  I use X-pro cokin filters and they don’t have a a 10-stop ND available.  Actually, no one does.  So I took some plexi, got it cut down to the right size and took it to the local tint shop and had them put some limo tint on both sides.  Works out to be about a 8 stop filter, but I can actually stack 2 for 16 stops.  It’s not perfectly neutral which would be OK, but right now it’s got a few different colors across the panel.  I’m hoping this is from the liquid they use to apply it as they’re still brand new but only time will tell…