AuthenticPassionfruit644 that's pretty much up to the discretion of the immigration officer, no matter what kind of visa you have. Holding a visa is not a guarantee of entry.
For the price of these onward ticket websites, you could just buy a real flight with Air Asia to many destinations.
Thai immigration aren't stupid, they know that the "onward ticket" isn't a real ticket. But you can buy an actual ticket from BKK - Phnom Penh for 1500 baht and then just never use it.