Probably will work for Immigration but it is your airline that normally asks for that proof so check with them or you may not be flying without a flight ticket exiting Thailand
If you are on a 12 month Non-OA Visa which has not yet expired (the Visa not your entry stamp) you can get new 12 month nsurance and border bounce to get a new 12 month entry stamp (but not multi-entry when the Non-OA is expired)