Theft-resistant seat/saddle bag?

Is there such a thing? My big, expensive Rivendell “Sackville” bag disappeared recently. Until this one, my bags had all been cheap junk, big enough to hold a shower cap (seat cover), an old handkerchief, and a couple of peppermints.

I’ve got security screws and security skewers so critical parts (saddle, handlebars, pedals) no longer disappear from my bike, but I haven’t secured EVERYTHING.