Coversion and Base Slab Trimesh

[Michael McCafferty]

Wondering if you will be able to help. I currently have a drainage network modelled using the drainage network editor and now the client wants us to model the MRWA conversion slab and bases (see attached, screenshot from MRWA drawing 200231-096-4) within 12d as a trimesh around the pit.

Is there an easy way to do this in 12d, as I am only aware of modelling the lid as a trimesh and amending the drainage.4d with them? Is it possible to model the conversion slab and the base too?

Thanks in advance