As stated above. A lot of ingame knives have simmilar dimensions to the Combat knife. For example the Hunting knife, and even the tiny cutlery knife (well, technically). In terms of size, I would even make the can opener attachable to boot.
Also, the bayonets (M9A1 and the AK bayonet) are similarly sized, and should also be attachable to boots.
One might also assume that more boots are able to hold a knife, such as "Combat boots" or "Jungle boots". But this is secondary.
Of course, the cleaver, cooking knife and stone knife need not apply for this.