Certificate of completion
After volunteers complete the series, have them write or record a summary of what they learned and specifically how they will apply the principles to their future teaching sessions. Prompts for that could be included in a reflection section at the end of each episode.
No matter how you do it, make sure you give them a certificate of completion for that series. It will be a reminder to them of what they have learned by participating in the on-the-go teacher training.
Advanced training podcasts
First, focus on the specific training needs you have identified. Survey class participants to determine if there has been improvement in the quality of teaching and learner engagement.
If all goes well, according to your standards of evaluation, you could consider another training series that builds on the first one.