I haven't made a lot of monkey fists, but the way I've been making them is by constructing a two-bight knot to serve as the core, and tucking that inside the four-bight knot. This is all done using the same length of rope, which is a little tricky. This is pretty much the method Matthew at Flamma Aeterna uses, and he documents it here:
www.flammaaeterna.com/t-tutmf.aspx
(I use a slightly different method for rigging the connector, but nothing significant).

One pitfall I've discovered is that making the knots too tight shortens burn time. Making them too loose obviously doesn't work.

Instead of a kevlar core, I use a wood ball. This lets me get the attaching hardware just right on them. Burn time seems fine.

I have seen the wood balls eventually burn and the connectors dis-connect...Not ideal.

I prefer an absorbent core. Either make a kevlar core like previously described or.... I made a pair with towel rolled up in a ball (about 3/4 to 1 inch in diameter) encased it in wire, (wrapping it round and round it) then created a strong loop out of wire to use as an attachment point (similar to the ball with a screw eye that most people use, except it's all one piece, and the towel acts as a fuel resevoir....I get a good long burn time out of my tiny 3 byte monkey fists.... (Im a 5 footer woman and don't want really heavy balls U know...) and I've had other spinners comment at festivals and such that I get an incredible burn time out of such tiny poi....

Hope that helps, use a decent and strong gauge wire, but coat hanger wire is too thick and not flexi enough.

Home of Poi are made from two cubes of wood that are then glued together sandwiching the hardware nut. I have had zero problems with damage to the wood as it is completely covered.