Good stuff on the build
Andy,
I just got in due to the CNY celebrations of eating and gorging on mandarin oranges.
To cut vertical 90 degrees on foam I would raise the foam slightly higher off the table with spacers, extend the blade and rest it flat against a raised ruler such as your steel blocks and slice in a sawing motion. The blade has to be angular type and new like this...
...so that it can easily slice into the foam board with the sharp tip. The space between the foam and table top lets the blade cut clean through and assist with the sawing motion. The steel block acts as a guide to keep the blade at 90 degrees.
No matter how small the apeture you cut you will need a ruler for straight cuts.
Anything new learned can be applied to enhance the enjoyment of the hobby.
Cheers,
Richard