Hi, @pattypeep. I think it'd be FINE to mix up your own! I often do that when I don't have something on hand.
I remember years ago when I needed curry powder, but had none. Until I looked it up, I'd no idea that there's really no such thing, and that it's a Western invention!
Kind of like Americanized Asian food.
Anyway, I just looked up 'Beau Monde' seasoning, and it appears that the spice company Spice Islands owns the trademark to the name.
"What is Beau Monde Seasoning?". wiseGEEK. Retrieved 6 January 2015
So, I'd say go for it & just make your own version!
Muddling through...