My first reaction was Dry Ice

I used to mess with that stuff all the time as a kid. I could buy it at Ice Cream Parlors. Be careful with that stuff as you can get burned real easy and not know it till it is way too late.

Would the ambient temperature reach -50C? I don’t know enough physics to know how that would work…I would take a mini-insulated lunchbox thingy and put the dry ice in but would the ambient air temperature inside reach -50C?

I used to dry ice stuff all the time and take a shoe box, line it with foam and wrap dry ice in news paper and put a layer on the bottom and sides. Put your “box” in there and simply cover it with small chunks of the dry ice and cap with another sheet of dry ice and it should get to -60C no prob. At least that is what Wiki Says…
“At atmospheric pressure, sublimation/deposition occurs at −78.5 °C”
so to me that means the solid dry ice is much colder than the temps you need and you may need to insulate your block to control the constant core temp of -50 C you are attempting to achieve. Crude but very doable in my book! Save any left overs for you Halloween party and throw chunks in your drink for that festive foggy/mystic look!

@Cruiser is right. Dry ice is the way to go. It is cheap and fast. You won’t be able to maintain the -50 C as you will pass through to -78 C. Use a thermocouple to see where you are then calibrate to that value.