Assembly manual:

First print the spring of your choice (there are two versions, perfectly stiff and extra stiff.

Note that the springs have bumps on one side of them, in this picture shown with a red line:

Then prepare your toggle switch and insert the spring inside with the bump facing the buttons:

Note that the spring might not want to go there so you will have to bend it using some force (don't use to much force, the spring is not indestructible)

 after that it should fit nicely.

Your modification is done :)