I personally prefer these. It's foam so it's softer. It has an adhesive backing and can be sanded to make the edges rounded so they contour to the inside of the boot.
1/8' BOOT FOAM- 10'x10'
I've used these by creating my own insole shim, cutting side shims for the toe box, and right now I have a piece on the top of my liner over the toes to close some space. They're versatile and easy to work with and relatively inexpensive. It's hard to find a boot that fits your foot perfectly. Don't fret too much about them being a little big. This is a pretty solid solution if exchanging them isn't an option.