Procure a polyethylene transparent plastic envelope. Sites like KellySearch.com offers a list of where to purchase such envelopes, or try browsing sites like ClearEnvelopes.com and BlockHeadStamps.com. These envelopes are manufactured by assorted companies as flat tubing with varying degrees of thickness and diameter, so do some research to find out which brand best suits your needs. For observation purposes, Powell recommends using a tube envelope that is an inch or two longer than the dish.
Cut the plastic tubing to desired length if needed. Insert the inoculated Petri dish into the tube. Cover both ends of the tube temporarily with cellophane.
Seal both ends of the tube with an electric flatiron. The cellophane will protect the plastic from melting as you seal it. Colonies can either be studied through the plastic or taken out for observation, then returned to the tubing and sealed.