What software do you have available? There’s an easy way to do it in inkscape, which can probably be done in illustrator etc.
Basically you measure the width of one of the slots.
Say it’s .125” and you want the slots to be 0.03” thinner.
Now select all.
Be sure the aspect ratio is locked (the padlock between the width and height)
Now change the width (or height) of the selection: add “*0.122/0.125”. This will resize your model so that your slots will now be 0.122”.
Make sense? Widths and heights can accept equations, and you’re basically multiplying by the desired width of your slots and dividing by their existing width.