How to turn not solid object into a solid?

Im having trouble making this a solid and was wondering if anyone knows how? I have to turn it into a waffle model and i think in order to do this the whole thing needs to be a solid. any help would be appreciated
chair.3dm (4.0 MB)

A solid in Rhino is a closed surface or polysurface with no naked edges. It also needs to be a manifold surface which means that there can only be two surfaces meeting at an edge. Naked and non-manifold edges can be found using the ShowEdges command.

What is the design intent of the open “seat” surfaces? Do they represent think sheets? If so you can provide thickness using the OffsetSrf command with Solid=Yes

Or is the design intent for the “seats” to be closed at the ends? If so you will need to create additional surfaces and join the surfaces together into closed surfaces.