Along with Many regular sized Doctor Who LTC's, I also have a few smaller cards.
"Tardis Ring" and "11 Doctors" are both 1"x2" and are sometimes folded.
I try to place a Letterbox in each state that I visit. Large test tubes are great containers, but the stamp needs to be long and narrow. I carved some state names to plant while on an 8-day 7,000 mile road trip last year. (I never stopped in Indiana or Kansas)