How can I extend my stay in Thailand after my Non-O B visa expires without needing a work permit or sponsorship fees?

Good evening,

I am a volunteer teacher in a migrant school in Mae Sot and returned again to Thailand on April 21 with a 3 month Non-O B visa, which then expires on 19 July 2019. I also have a work permit, valid until October. This was organised through a Thai foundation, with an appropriate donation, which I was quite happy with.

However as of the end of June, I will not be volunteer teaching in Thailand any more. I would like to stay (until I had planned to return to Australia in December) but want to avoid paying any further fees for "sponsorship" and work permit I don't need.

So I would appreciate guidance on how/what may be the best way/visa to do that.

Thanks in anticipation. :)