I would think that perhaps two shorter ads (10-sec), at roughly the 20 and 50 minute marks, would be better than one longer ad.... and please make them different each week, with a different panelist reading them!
I am concerned about the influence of advertisements shaping the message. This is one of the reasons I think mainstream media has degraded to its nearly worthless status. Their goal has become to harness ears/eyeballs, which can be sold to their sponsors.
Other ideas:
1) I think a better approach would be a limited plea for donations. Have a random panelist make an unscripted ~10 second plea, once during each podcast, at a random time, and provide a URL where we can go to send money.
2) A donation plan (one dollar per month, for instance) would be a painless thing to sign up for.
3) How about an SGU credit card, where 1% of all purchases (or a user-selectable percentage) go to fund the SGU? Make it cool looking, and it would make an easy way to proselytize the "good word" of SGU every time you make a purchase.
4) If you go with the credit card approach, perhaps different percentage (and accumulated contribution) levels could come with special benefits. I'll let you brainstorm on what would be cool benefits for different levels.
Ok, thats all I've got for now. Good luck!