Having had rebuilt my Milling attachment. I just got around to making a set of Jaw Blocks. (Were missing when I bought)
I did not have any real specs to go by. So, I built them (2 sets) to my own also based on my selection of materials. The sets can be oriented in many ways.
I choose Aluminum for several reasons. 1: I seem to cut more aluminum than steel. and the Aluminum is less likely to scar what I may be working on.
Also, I created a plan sheet PDF if your interested in what I built and the sizes. AtlasMillingAttachmentBlocks Or see bottom pic.