Adding a 3rd buckle to spk's

This is the reason its better to buy a smaller boot than bigger if you are unsure. You can at least have work done to make some more space in a boot which is tight. Obviously the best option is to get properly fitted to prevent either of these scenarios.

If you really want to work with what you have, perhaps try some J bars, heel wrap ect to take up some space around your ankle. You can't just add a buckle.

Have you checked to see if the shell fits? A boot that is too long/wide/high volume for your foot can still feel fine while just wearing it around if you have the liner in. Put your bare foot into the shell without the liner and follow the instructions in this thread to see if the liner properly fits:

Now, if the liner does fit but you're still feeling that heel slippage, there are a number of small fixes you can do to help. Footbeds, or the addition of a higher volume custom liner (like an intuition) may solve your problems, or you may have to resort to something that I don't know quite enough about bootfitting to reccommend (I would take it to a shop and see what their bootfitter has to say). If the liner does NOT fit your foot, you can do all the little fixes/add-ons you want, but the boot will never fit your foot perfectly. You will be able to get close, but there's only so much you can do with a shell that's too big for your foot, and you'll never get the same fit you would get with a shell that matched your foot perfectly. Hope this helps.

-Isak

A custom footbed will usually make your foot smaller so it probably wont help in a big boot, but if you get something like a sidas which supports your medial arch, it can raise the instep which could help keep your heel down. The arch will also stop your foot slipping forward in the boot.

But as has been said really if the boot is too big there is nothing you can do, and you will be always having to get them refit to just try to keep your foot still in the boot.

I would personally cut your losses now and stick them up for sale. Hopefully you can get what you paid as the condition has not changed since you got them.

Take that money and go get fitted up properly. It will be cheaper in the long run and you will have boots which actually fit, so chances are you will ski to a better level as you wont be being held back by the boots.