assuming the boots fit well
replacement aftermarket liners
or budget
I found a good way to take up space, in a neutral fashion OR in specific areas - sheet of thin adhesive backed 'felt'
easily gotten at MIchaels, the cratfs store - about $1.25 ea
thickness of these 12x12 sheets is about >1mm and will 'flatten' to a stable <.5 mm or there abouts - so nice, small adjustment possible
liners will 'pack' more where there is greater pressure, so these felt sheets can be cut to shape or can cover a general area
for overall tightening.
adhesive is not permanent, so easy to replace/change
For forefoot areas where there is a possibility of 'shifting, due to liner re-insertion into the boot shell, I'll cover the leading edge of felt piece with some thin packing tape to prevent shifting upon liner insertion.
helped a friend last season who has broad feet, but thin lower leg above the ankle. Wrapped one form-cut sheet around the liner upper, above the ankle. worked a charm - gave a snug, even fit for the boot shaft/uppper.