After building and testing the robot, I have some changes to consider.
First of all, the plywood I used is not great for the legs. Surprisingly, it's not the weight of the robot resting on them that is the issue, but the fact that the parts twist, especially when the robot tilts from side to side to unload its leg before raising it, and the whole thing becomes rather wobbly.
Now, what are the alternatives?
I could go with stronger plywood (the one I used was sold together with my scrollsaw, and is rather soft), and perhaps laser-cut it this time. Or I could go with laser-cut acrylic like in Fluffbug, it should still be stronger than this.
Or I could really go all the way, and use 2.5mm aluminium sheet. It's soft enough to cut by hand with a hacksaw, and should give me plenty of strength. I'm tempted to do it, especially if I can cut it at the hackerspace and avoid the mess at home.
In either case, I think I will also need to make the legs a little shorter. I was optimistic and really scaled them 200%, but that makes the robot look really tall and narrow, and doesn't help with stability. Shorter legs should make it easier to fine tune the gait, no matter what they are made of, and this is not a race, so I don't really worry about the speed.