I use the Bic Metal guard single blade disposables when I can't use the shavette or the DE. They do the job rather well and comfortably and are better when compared to anything else I've tried (disposables/carts). Ymmv.
I've been going through this same deal, as I am getting ready to travel where I don't want to check a bag (and I didn't get my act together to mail blades ahead of time). I looked at some disposables and other options, but in the end I just picked up another pack of cartridges for the M3 I haven't picked up in over a year.