Concave shape collider?

Hello,

I have a concave, bowl shape (Upside down bowl) but I'd like the player to be able to go through and underneath, but if I add a primitive collider I can't without setting the collider to "IsTrigger". The object has a rigidbody component so without an appropriate collider it falls through the terrain.

Could I parent a very very thin circular plane to the bottom with a collider, would this stop the entire mesh from falling through?

Is there any way I can add something to stop it falling through the terrain yet allowing the player to pass through?

I'm trying to simulate a bubbleshield from the Halo series, if anyone is aware of the game mechanic