Hi Keith - shell is probably the weakest function in MoI currently, it's a very complex operation since any surfaces that do not meet up smoothly will not have offsets that exactly meet up and they will need to be extended and intersected with each other. The geometry library that MoI uses doesn't have a very good mechanism for that. It can work for simple cases but with 392 faces in your model that's a lot of extensions and intersections to be calculated. Then on top of that you've got some small sized features and surfaces with tight bends in them and if your shell offset distance is larger than the size of those small feature that's an additional difficulty.

You might try bringing it into something like OnShape which has a much more sophisticated shell function, but it's probably going to be difficult to shell anywhere.

You could probably make some progress on building a shell by using Edit > Separate to break some of the large surfaces out to individual surfaces and then use offset on those surfaces to build some pieces and then work on filling in the gaps.

- Michael