Hi Leonard, until I figure out what is going wrong with chamfer in this case, probably the best way to complete this would be to use a combination of Offset + Loft instead.

To do that, you go through those current curves you have an offset them to the inside by your chamfer distance, like 0.25 inches I think you were using.

So that gives you this:



Then you move one set of the curves backwards by 0.25 units, so that they are staggered from one another like this:



Then you go through and select matching pairs of curves, and run construct / Loft (with cap ends option off) to build the chamfer pieces like this:



After those are all built, switch to the Top view and do a window select to grab all the curves for the front part, like this:



Then run Construct / Planar to build a planar cap surface through those.

At this point the front piece and the chamfers are all built, you can join those together.

Then mirror it to make the back part, (this much is in the attached 3DM file) and then to complete it you'll need to select pairs of edges and use Loft to build the side wall pieces which then can all be joined together.

This should at least allow you to finish the model with your originally desired sizes.

I'm not sure yet why Chamfer is having so many problems with this situation, I'll see if I can figure anything out about that but that may take a few days or so before I know any more about that.

- Michael