What should I do if my volunteer visa in Thailand is about to expire and my foundation cannot renew it?

Hard to say Mark. At the very least, if you are unable to renew your visa in country you'll need to cross over to say Laos and spend like a week over there before trying to come back. Perhaps come back at a different border to the one you exited at. Make it look like you actually went to Laos for tourism.

If that fails then you may need to think about going back to your home country for a bit or remaining in Laos. It's easy for foreigners to use agents to extend their visas there. The situation in Laos is similar to Thailand 20 years ago. Basically anyone who can afford the agent fees gets an extension. They make the documents for you, you don't have to show thousands of dollars in your account transferred 2 or 3 months before the date of application as in Thailand and Laos doesn't require 90 day or 12 month reporting either, except for the purposes of renewing your visa which is generally required on an annual basis.

Any other border is OK, such as Chong Sangam or Chong Chom.

Cambodia is OK just avoid Aranyaprathet on the way back.

Just spend at least a night or two in Laos or whatever country you're exiting for. It's not that hard. Problems arise when people bounce and come back the same day. Why risk it. Stay away a few days, make a trip out of it. Besides, a bit of a change of scenery from Thailand is always nice and Laos is absolutely fantastic. Hardly anyone wears a mask either, unlike in Thailand.