For added strength in special uses modules can become rigid with the addition of a bottom skin.  Much like a hollow door it becomes very stiff.  The addition of access holes does not diminish strength.

Shown: Special raised arch in the subframe raising bottom  of layout decking for work table clearance

Travelling corner module needed extra strength for heavy use:

Over-length /over-width module needing stability:

Corner module needing anti-flexing support: