The bottom bumper helps keep the seat from hitting the crossbrace if it slides down. The plans call for lag bolts to secure the front cross-piece to the front support. If the tread is applied to bare wood it's almost impossible to get all the old glue off. The woodscrews used on my seat are brass. The brass screws also look better.
Original plans suggest using a piece of cardboard as a spacer on the seat support block.
I found that a piece of thin plywood panelling works better. I found it easy to cut the foam for accessories using a sharp butcher knife. The eyepiece holes were cut with a plumbing drain tube extension. The edges of the drain tube were sharpened slightly, then rotated and pressed into the foam.