Hundred Second Theater is now accepting script submissions from writers. Scripts may be edited to fit the 100 second length requirement. Since there's no revenue, you won't be paid. You'll be credited on the episode graphic, and on the youtube video edition, but the audio file itself doesn't usually contain credits (due to the limited number of seconds available). You retain ownership of your script but you grant a perpetual license for this production of it. Since the productions are licensed permissively (CC BY-SA) you can distribute it and include it in your portfolios.
In order to work out to 100 seconds, your script should probably be 300-400 words, though that'll vary depending on a variety of factors (how long the effects are, how many of your words are directorial notes, whether some of the lines are in the background or characters talk over each other, etc).
Please provide your script in plain text. See some of the existing episode scripts for preferred style, though a different style isn't a big problem.