Foam core board is great stuff.
I work with it every day.
A step up in durability (with the same construction concept) is Gator board.
Both the board and sanwiched foam is a little sturdier.
Not sure if is available in 1/8" though.
For 99% of my foamcore cutting I use a snap off Olfa blade.
The X-Acto is for thin stuff and detail work.
Plexi would make a super template.
Just a word of caution for those that aren't familiar with cutting a long a plastic edge (plexi template or triangle)
The knife can & will grab the edge of the plexi. There is very little resistance and the knife will speed up and skate where it wants once it is on top of the triangle or template.
When a new guy comes to the job we all compare scars and 'knife wreck' stories.
Same goes for inexpensive aluminum rulers, always go for steel.
I don't make enough of the same sheath to warrant templates, so mine are all still in the manilla folder stage.
Hope that helps.