There are a lot of things about the right shape. Thick dowelling may be strong enough. Alternatively, you could braid thongs of leather into the right shape.

I once talked to a Domme who used eggs to gag her sub. Find an egg the right size to fit, lodge it behind the teeth. Threaten sub with punishment if they break the egg (so they have to be very careful of it) then tell them that the only way to get the 'gag' out at the end of the session is to break it - so lots of mess and humiliation and, if you are really evil, the punishment you promised for breaking it