Why is an enclosure not suitable when running the analysis in Mechanical?  What problem does it cause?  Isn't it just a solid?

If you don’t like the solid the enclosure created, add a component, select the solid pipe and type Ctrl-C, Ctrl-V which will put a copy of the pipe in the new component. Add a sketch and draw the concrete block and pull it into a solid. Then use the Combine tool to subtract the pipe from the block. Finally, delete the solid inside the pipe.