Griz,
I used braid for bottom bashing a couple of years back at Fitzroy. I used a knot I found that was listed in the spidrewire booklet. It never did let go.
Basics of it are:
Double your braid and pass it through hole.
Do 4 loops of the looped end around the doubled bit. Same as blood knot.
Pass the looped end through the hole you've made and then pass the loop over the swivel.
Pull it up slowly and evenly.
Hope that doesn't confuse you too much.