I finished making the case tonight.  It didn't turn out quite as well as I had hoped, but it is still OK.  The biggest problems I had were getting all the styrofoam layers cut to the same dimensions and getting the hardboard shell in place and fitting.

First, here is the finished product.  After that I will walk through the construction process.

Looking at other cases online, it looks like lots of people make cases for electric guitars, but not many for acoustic.  (Or, if they do, at least I couldn't find them).  Also, lots of people seem to use plywood for the case; I'm sure that works well from a protection POV, but I didn't want to require a cart to move my guitar.  So, instead of building it out of plywood, I used styrofoam with 1/8" hardboard for an outer shell.

I used some sheets of styrofoam insulation panels, 14.5x48x0.75" in size.  These I cut diagonally (as this was large enough to fit my guitar, and would result in much less waste).  I then hollowed out the sections as needed, using a blow torch to heat a coat hanger.

I then glued the layers together one at a time, inserting the inner fabric where it needed to go:

Once the inside foam + cloth was complete and verified to fit the guitar, I cut 1/8" hardboard and glued it on the sides.  (I forgot to take pictures of this step).

Finally, I used spray adhesive to glue some more fabric on the outside of the case.  I tucked the corners into the case (between the hardboard and the styrofoam) for a nice clean look.

Included in the case are three pockets: one circular one for a humidifier, and two rectangular ones for whatever you want to include (capo, picks, etc)

After the outside material was glued on and tucked in, I added hardware - 4 hinges, 3 latches, and a handle.  I put the handle at the center of gravity (with the guitar in the case) for easy carrying.  The latches and hinges were just added where they looked good.  (I had put double thickness hardboard on the places where the hardware would attach, to ensure that it would be strong enough).

The case is the perfect size for the guitar.  Looking back, I probably should have made it slightly larger so that it would fit in the case with an attached strap a bit better, but oh well.  It *can* fit with a strap, it is just a bit finicky.

Total weight with guitar is just under 15lbs (less than 7kg).  It is not going to be as strong as a behemoth made from 1/2" plywood like some of the other ones I have seen online, but as long as you are reasonably careful with it, it should work quite well.

As for the issues with this case, it is hard to see from the pictures, but some of the corners are not quite aligned (due to the problems getting the styrofoam and hardboard in exactly the right places), and there is some hot glue showing on the inside.  It is definitely not up to my normal standards, but this is a first time doing something quite like this.

If I were to do it again, I think that I would make the shell first, using 1/4" plywood for the sides and 1/8" hardboard for the top and bottom.  Once the shell was made, I would then cut the styrofoam and place it inside.  (This is the opposite of what I did here - I started with the foam).  Oh well, live and learn.