Rather than add the same amount to each strip you could add the same percent. Take your narrowest and add a half inch. Divide one half by the original size. Whatever you get is what you multiply each strip width by to get what you add.
For example. If your narrowest width is 1.5 in. Take .5/1.5 =.33
Next width is 2 inches. 2 x.33 =..67 which is about 5/8 of an inch so you would add 5/8 to this strip
This would keep all the widths in proportion. To keep the whole thing in proportion you could also do the lengths.
It's a lot of math but would not distort as much as adding the same amount to each.