Can I extend my Non-B visa after my work permit has ended?

It all depends now on you and your (former) employer. If nothing was officially terminated - you might find a good way to fix things while avoiding overstay. Fix it now.

You're on an extension not a visa, and the moment you stopped working your extension and work permit became void.

For every day you're here since that date you owe 500฿ as you're overstaying.

To my understanding, you cannot be on a Non-B "Visa" anymore. The visa was valid for 90 days - so you must have extended your stay based on work before. You say you don't work anymore ... if the work is terminated, so needs to be the work permit ... and with it, your extension and permission to stay. From the day the work is gone, you technically only have 7 days to leave the country. You might technically thus even be overstaying. Certainly no way to extend stay based off of work, if the work is terminated already. You will have to leave Thailand and re-enter on a different visa (or visa exempt), imho. I would rather go for a visa in that case, because of your history. But maybe someone with more first hand knowledge on this can chime in here.

To avoid unwanted consequences, I would also advise to speak with your (former) employer how to end administrative things properly.

You are not on a Non B visa anymore. You are on an extension of stay based on working.

The date of your work permit is irrelevant. The day you stopped work is the day your reason to stay in Thailand stopped. You are on overstay since that day.