Thanks mrtn. You're right, that was easy. I just used the original bolts and even though they are pan heads, the plywood is so soft they made perfect circles. So now I've drilled the holes and they match exactly. But the reused bolts from the seat brackets are too short to reach the holes through the 3/4 plywood. I thought I checked it but I guess I didn't check it very well. I countersunk the bolts so they wouldn't protrude and even then I only get a turn or two of threads. How deep a countersink can I make before I compromise the plywood too much? I thought it might be easier to just buy some longer bolts, but I'm having trouble locating a place that sells them.
Or maybe I'm just not looking for the right thing. I know they are T50 torx, but it seems the T50 refers to the driver, not the actual bolt size?