Wow, if I had any clue that I was going to be doing this sort of thing later in life I would have paid more attention in Geometry (and drafting) class! So, it turns out that there is a lot more to cutting a bastion than nice straight walls. The intricate inside corners of angled walls almost defeated me (BTW, Pythagoras, you were no help) but then it suddenly occurred to me that I didn't need to cut the object in one continuous action, all I needed to do was make a pile of parts that would reassemble into a bastion. Blunt Force has always been a specialty of mine so the world made sense again and I got to work.
The next step was putting the lines down on my 1/120 scale test model, this is where some drafting skills would really have helped (or a CAD designer and a 3D cutter). After much head scratching and a lot of graph paper the drawing came together and was transferred onto the blue board. There were a few issues during the transfer process but I got it worked out producing this magnificent mess of a drawing;
the red lines indicate the footprint of the bastion,
the lines labelled "cut" account for the inward tilt of the wall face
As each cut passed completely through the block of foam I glued the block back together after each cut, making sure to only glue the areas that would be part of the final piece. This way I was able to use the fence on the foam cutter to ensure that the lines were straight. Finally with all the cuts made and the spoil cut away I had this;
now all I have to do is figure out how to get something three times as large to go through Proxie
All in all a satisfying experiment. Now that I have figured out the way to proceed with the cutting, reassembling, more cutting sequence I just need to find a way to get the parts through the tool. Another consideration is that each bastion will have a largest dimension of over 24" with all the storage and handling issues that arise from that.
More thinking may be in order.