You can put a little scotch tape on the contacts of your spare battery to save some room.

I have two methods I've used to prevent accidental activation of a Bic: one is a zip-tie exactly the width to fit underneath the gas button, cut off. The other is a short piece of relatively-large diameter shrink tubing, shrunk over the top of the lighter. Either way it prevents accidental loss of fuel while still being something you can remove in the field.

Does this kit assume that you have something to carry water in? There are plastic bags that hold water securely and fold-down very small that you might consider.

What's your use case for the lockpicks? Some places you risk arrest by carrying them, even where there is no illegal intent.