J bars, c pads, butterflys, etc, all those foam "solutions" ultimately suck. They'll work temporarily but they're going to pack down, forcing you to add more foam and causing potential nerve damage in your foot since the liner is no longer hugging your foot uniformly (usually adding excessive pressure around the ankle bone).
If you're forced into a situation where you need to put foam in your boot it's because you have a bad shell fit. When you buy a boot it should be snug around the narrowest part of your foot. If it's too tight in other areas that can easily be fixed with a boot punch or boot grinding. If your shell fit is too large however, nothing can be done to shrink it, you just need to get a new boot.
I have a narrow heel and medium sized forefoot, and I sized my boot for my heel and things have never been better. I'm in a 95 last race boot and my boot fitter just boot punched the hell out of the forefoot and now I'm in the best shell fit of my life. It's also important to note that some low-mid range boots have nothing but a tube shape for your ankle/lower leg. Higher end shells will be in a tear drop shape instead of relying entirely on the liner to get your heel hold.