I finally marked up some photos with measurements. Hopefully these are fairly clear and helpful.
The top of the seat:
View attachment 800467
3/4" plywood cut 16" wide x 9-1/2" deep, with what looks to be a 45 degree rip down each side (I didn't measure the angle exactly). The result is the seating surface is 16"x8". The bolt holes are spaced 6" o.c. On the long axis and 3" o.c. On the short axis. Bolt holes are centered 2-1/2" in from the long edge of the top surface and 5" in from the short edge. The holes are 3/4" diameter and have a 1/4-20 flange head bolt threaded through T-nuts on the underside. These form threaded studs which are used to attach the seat to the frame with nuts.
The underside:
View attachment 800451
Constructed mainly of 3/4" blocking. There are two pieces, 14-1/4" each, on each of the long sides. One is in the corner between the main seat surface and the "vertical" side of the seat. The other is at the bottom edge of the vertical piece to form the "underside" lip that the covering is stapled to. There is a single piece of 3/4" blocking 6-1/4" long centered on each of the short sides. The next photo will add some context.
The profile:
View attachment 800456
Here you can see the main seat surface and the 9-1/2" overall length with the angled cuts to make the top face 8" wide. You can also see the short-side 3/4" blocking mentioned with the previous photo. The vertical sides of the seat are also made of 3/4" plywood, cut 16" long and 2-1/4" wide, with another 45 degree rip so the outer face is 1-1/2" wide. This is butted to the top piece to form an angled face, then a straight vertical face, and another angle that wraps toward the underside. A piece of 2-1/4" long 3/4" blocking is placed behind the vertical face for additional structural reinforcement. The long pieces of blocking from the underside pic are also visible here running down the length of the seat. One piece of that long blocking provides the bracing for the corner where the vertical piece butts to the top piece, and the other one provides the underside lip for securing the seat covering to. Now it's clear why those long blocking pieces are 14-1/2" long; there is a piece of 3/4" blocking placed perpendicular to the long blocking at each end, thus making an overall length of 16".
Another shot of the underside, showing the placement of the long blocking pieces to form the structure of the underside, and the aforementioned blocking on the short side that provides backing for the vertical piece as well as the short side of the seat.
View attachment 800458
This last shot shows the short blocking pieces and also the placement of the 3 screw holes that attach the side pucks to the main seat. The side pucks were also glued. The assembly was done with square-drive cabinetry screws like you'd typically see used with pocket holes.
View attachment 800459
One hole goes through the "short side" blocking, and the other two are in the gap between the two pieces of "long side" blocking on each side of the seat.
Happy to grab anything else anyone needs.