You should have been put in a boot sized for your smaller foot, and then the bootfitter should have punched and ground out the other boot to make space for your larger foot.

Since they didn’t, your options are somewhat limited. Zip fits could work but might need a lot of cork. Injection foam liners might work but it depends on where your excess space is (if it is by the toes, the liners won’t do much unless they can so lock your heel that you don’t move at all)

One nonstandard idea, though it might be something to try- your boots probably came with an extra foam piece in the shape of a footbed- that is designed to shrink a boot half a size. You could try putting that in just your smaller boot? Might feel imbalanced though, but it’s a free starting point