I found a site with plans in Spanish (I think). Anyone know of foamcore insert plans in English before I get brave enough to try and make one on my own?
I'm assuming you found Paquisto's plans. I built an insert for Mombasa using his plans. It's easy to translate his page to English in Google Chrome. His foam insert for Mombasa is the best, IMO.
On his site, you'll see a download link for some files that open in Sketchup.
You can get all the measurements you need right in Sketchup. These plans are language-independent and intuitive.
It looks like this when opened: https://gyazo.com/6d1e1917cdec81f870b4ffa022f0fd7e
All you need to do is cut out all those individual pieces and glue them all together. You'll see each strip is 23mm wide. I suggest cutting at least four 23mm wide strips the whole length of your foam board, then cutting those down to the proper length.
Then cut three squares 288mm x 288m. One of these squares will stay whole, the other two will be cut up to form the different sections of the insert.
an important thing to note: Paquisto's insert does not accommodate sleeved cards without a slight modification.