In order to measure and transfer the correct angle for the joint support of the sloped back track, I used one of my more esoteric tools, a sliding t-bevel. This tool is handy for taking an angle and either marking something else or setting a saw angle. I did the latter to produce angled cuts that precisely match the slope of the plywood. I have no idea what the actual angle is and I don’t need to.
On the simpler end of the tool spectrum, I picked up (no project is complete unless I get to acquire at least one new tool) an inexpensive plastic adhesive spreader on one of my trips for yet another length of 1×3. This turned out to be a great deal! In the past, I have used scrap bits of wood or popsicle sticks to spread glue over large areas. It worked, sort of, but there always seemed to be too much or too little and my fingers got goopy. The spreader lets me quickly and evenly apply glue which reduced some of the anxiety about glueing down the main Homasote piece. I plan to get even more mileage out of it when I glue down ties.