Turns out my daughter had some left over ram board from some craft project she worked on. Not only that but I had all the correct lumber I needed in the barn already leftover from some cabinets I built. Turns out 3/4" ply plus two 1/2" ply plus two sheets of ram board hit 1.75" right on the dot. I got everything cut to size and then started working on tracing out the templates from the plans Bingham provided so I don't have to cut into their plan sheet. Hopefully get the form built tomorrow. Thanks for all the suggestions guys!