I also marked the outline for the well for the flywheel.
The holes were drilled 2 mm oversize, so it theoretically was able to take up any inaccuracies in my marking. The holes for the well were drilled to the exact width of the corresponding square hole in the base plate i.e. 18 mm wide.
I brought the foundation to my small drill press, and decided to use some of the Forstner drills I purchased for another project some time ago. I have a feeling that I bought those drills too cheap, because they aren't very good. But they have been adequate for my use so far.
Some way I managed to malposition 5 out of the 6 holes which is quite a lot below my normal standard. The problem was that I didn't measure, but used the actual base plate to lay out the locations. This was not possible to do super accurate, and I made myself believe that it would be OK, but it wasn't. It doesn't matter a whole lot, since I could make the holes fit by chiselling out where the screws touched the wood. And nobody will see it unless I take pictures of it or they take out the steam engine from its display case. Which is unlikely to happen.
Gustav wanted to make some holders for obstacles for the riding course, he likes to jump on the pony, and I think it is a good idea to keep the hide glue fit.
The holders are made out of some 2x4" larch with some small pieces nailed on to hold the boom (I don't know if it is the correct word).
I helped him by sawing out the pieces so he could concentrate on measuring and marking the wood. He then drilled pilot holes and nailed on the small holders. He learned about leverage, since he had to use a bolt cutter to bite a bit of the nails so they wouldn't protrude on the back of the post. At first he tried to bite the nails holding halfway on the handles, but that was hard. Using the full length of the handles helped.
The foundation with the holes drilled (the upper right one had the correct position)
The engine base plate fits nicely in the recess.