Capping a 3D object after trimming

 From:  Michael Gibson
4108.2 In reply to 4108.1 
Hi Mark, the Construct > Planar command can cap off planar end openings:

It's a multi-purpose command - it can be either applied to a selection of planar curves to make a trimmed plane surface through those curves, or it can be applied to a selected surface object to cap off planar openings there as well. When applied to a surface object the caps that are created are automatically joined with the original surface.

So for example for your object, select it and run Construct > Planar and then those holes will be filled in and joined, and you'll have a solid directly after that.

- Michael