You have the right idea, but those won't take the wear in a figure skate, I've tried them, they peel right off. What you need is a heel counter (see
here marked d).
There is another option to modifying the skate. Modifying the skate is hard to get it just right. The option is a gel sleeve.
You can find gel sleeves at various places. I prefer a gel ankle sleeve. It's 3 inches by 10 usually and is basically a tube lined throughout with orthopedic gel. Don't get the ones made of fabric with gel patches here and there. They're not heavy enough.
The only place I can find that sells them online is
here. Other manufacturers (silipos, bunga) have switched to fabric sleeves with gel inserts which are useless for your needs.
Cut the tube in half. Slip one half on your foot so that it hugs your heel. You'll need to wear a thin sock or knee hi's over it to protect it. This is what I use. I go through a sleeve a year.
If THAT doesn't work. Go to a shoe repair shop, tell them your problem, and they may be able to glue in suede leather to reduce the space in the heel. Their glue will be much stronger than anything you can buy.
Next pair of boots, get split width.