First off, I take no responsibility of the destruction of yoyos through attempting this mod.
Anyway, here is how you hubstack a FH2. All you actually need is a jewelers screwdriver. This red one was made to accept a D sized bearing since I had ran out of small bearings. First, you start off by cutting this part of the hub. Do not cut down to the axle, the stacks won't be able to spin. All that is done is widening above the axle.
Widen the hole right above where the axle sits until it is able to fit the bearing, like so.
After that, to make the spacer fit tightly into the bearing, you fit the tip of a needle nosed plier into the spacer and tap lightly with a hammer, after a few taps, test to make sure the spacer fits tightly into the bearing.
After that, you have to replace the nut the yoyo originally came with with a shorter nut, which looks like this. The axle will still fit.
And there you have it!