I made Bill's shoulders with white melamine bathroom panel. This worked OK, but I had to soak the panel for a couple of hours in the bath. I also noticed that it began to wrinkle around the edges.
This time I used 1/8" hardboard, which turned out to work very well - it didn't require any soaking, and seems plenty strong enough. It was much easier to bend too, as I built Bill and Dirk single-handed.
Last time, I countersunk holes in the front of the arm box for the bolts, put them through and then covered the result with wood filler. Unfortunately I didn't sand them enough and the result shows through the paint. It's quite difficult to tell when you've sanded enough until you paint, by which time it's a bit late. So this time, I built an inner ball mounting for the arm ball and glued it to the inside of the arm box. Wood glue is really strong, so I'm not expecting that this will weaken it too much. The result is that the arm box has no bolt marks giving away my unreliable finish work.
This time I made separate armboxes, rather than working around the central strut at the front of the shoulder section. I don't think I can recommend doing it one way or the other. They both work fine. Doing it as one piece avoids the risk of getting them misaligned, though I don't know how much it would show.
Last time around I cut cardboard stencils for the shoulders, then carefully cut the shoulder board to exactly the same size.
This time, I cut the board about 1/2" to 1" oversize, attached it, then went around with a saw and belt sander to take off the extra material. Much less trouble, as it reduced the complication of making the board the exact size. If I'd got the tip in time, I would have pulled the nails after the glue dried and then filled the holes.
The slats were done much the same as last time...
With Bill I used Bocce balls for the arm balls. Unfortunately they are only 3.5" in diameter, rather than 4", so this time I have used Christmas decoration balls, as for the hemis. I've painted them on the inside to stop them scratching as they are moved.
I later replaced these with agricultural ball-cock floats from water troughs. These are very cheap and made of a thicker, softer plastic. They're also slightly oversized, but that hasn't mattered.
For the plunger arm I used several different diameters of PVC pipe this time, with dowel down the middle as before. The steel conduit I used with Bill is rather heavy, so I'll probably change that out in due course.