I assume you have a tank of CO2 or dry ice. CO2 is heavier than air... so a plastic bucket would work. The main problem will be the loss of CO2 by diffusion. A plastic bag around the bucket sealed with tape should hold for several hours once purged. I have never killed a spider mite so I don't know how long or how many treatments (to kill eggs).  I would start with a 4 hour treatment and repeat every couple days till they are gone.  CO2 isn't toxic to us but you want to do this in a well ventilated area because CO2 can displace the air and drown  you.