Cutting strips of balsa wood or EVA foam with a blade and ruler requires some care and attention. A good stripping tool makes the job easier and safer, producing more consistent and repeatable results.
Balsa strippers are nothing new and there are a number of commercial and DIY models available - this is just my version.
For context and a quick demonstration please watch: https://www.youtube.com/watch?v=6i9WbRWzRuM

The stripper has 3 main parts: the "handle", the blade holder, and the interchangeable spacer plate.
The thickness of the spacer determines the width of the cut strip. The spacer is slotted
to allow for different height materials (up to about 8mm), and is sandwiched in position when the assembly is tightened together.
Rather than employing specific blades this stripper uses commonly available utility knife blades, most any should fit.

From the the STL files you'll need to print at least one handle and one blade holder, plus one of each thickness spacer you require.

You'll also need two 4x16mm Allen headed bolts, with nuts to suit. Note: if you wish to cut strips wider than 5mm you'll need longer bolts (20mm).
Now, before you say "I don't have bolts and nuts".. do yourself a favour and buy some - they are super useful for projects and also as spares for your 3D printer - a variety pack of 200 or more (3, 4 & 5mm) can be had cheaply on Amazon or eBay.

Assemble as per the photos (4, 5 & 6).
Usage should be self explanatory but if you're still not sure just watch the video linked above.

Tips:
Bolts should be done up firmly but don't over-tighten or you'll distort the plastic.
The spacers are numbered (in mm) but if you print them in different colours you'll know the size at a glance.
A dull blade will not cut cleanly (or safely) - change it sooner rather than later.
Sketchup files are included, you can modify the 3mm spacer skp if you need a thickness not supplied (eg: 2.5mm, 7mm).