Life getting a little lean these days? Buying boots 2-3 inches to big doesn't make any sense unless it is the only way that you can see to get out on the hill.
If you are really serious about trying to make these boots skiable for you then you need to think about your problem a little differently. Don't try to hold the foot in place by putting something in front of it, This really won't solve your problem or make the boot skiable. Instead, hold the foot in place by holding it down on the boot board and back in the heel pocket by using material over the top of the instep and up the shin a little ways. A piece of a good quality camping pad should work for this.
Cut it long enough that it will lay over your foot from the balls of your foot to several inches up your lower leg. Next place it over your foot and trace it so that both ends are rounded with the end that goes up over your lower shin tapering in narrower than the end towards your foot. Taper the edges back so that the pad doesn't end abruptly. Then slip it in your ski socks being carefull to get it positioned evenly over your foot, and try on your boot. Keep it on for awhile and then remove and look for bright red areas. Trim these back a little and try again. A little trimming goes a long way so be patient and go slow.
The goal is to keep the foot from rising up off the boot board as you are skiing, keeping it in contact with boot board all the time you are skiing, while at the same time, keeping the foot nestled back into the heel pocket as you ski and move through your turns. The other critical area is the upper cuff fit. You want to have a nice snug fit in the top buckle are as well to keep the boot responsive to tipping movements. If it is too loose in the upper cuff try using material in front and to the sides of the and not in the rear of the leg which might push you too far forward. Again, be sure to ease the edges of the material by tapering it back from the edge. Good luck!!!
Ya know, I just have to respond to the Yahoo comment. Many might say the term Yahoo better fits the person trying to make a 2-3 inch to large boot fit, rather than a person who is committed to finding a person the appropriate sized boot and fitting it accurately.