P-Trap access box under basement floor

[Al S]

I have an under basement slab P- trap connected to the 4 inch main that then exits the house. Because there is a full bath 20+ feet from the trap, the pitch has the trap about 2 feet under the slab. The wood surrounding the trap has all disintegrated and the dirt is starting to fall onto the p trap. Any suggestions how to re-create an access box of sorts to get to the p-trap for clean-outs?

 

[Al S]

 

[Sponsor]

 

[Al S]

Forgot to show pit