For butters and moki, I would probably just not bother.  I find a good system is to buy a small collapsible kayak grapnel anchor and tie that to the end of the float line. Drop it in an area you are working, and take it with you when you move on.  Not many blokes get pulled under by monster butterfish and the freedom of not having a float attached is very nice. The anchor can hook just under your weight belt if swimming from spot to spot.

 

If you are thinking you would whack a fish bigger than say 20 pounds, or a small kingy, then attaching the spear to a float line is worthwhile.  Most float lines use a shark clip & loop system.  You can reverse it, with a shark clip on the float line end, and clip it onto the rubbber. The problem then is that the shark clip will slide down the 10mm rubber when stretched, and end up at the rear of the spear, which would be disastrous on release as your spear would have to tow the float and line a couple of metres forwards as well as the spear itself.  The solution is however quite easy.  Take your loop of rubber attached to the spear, and tie a knot part way down, so there is a loop you can put your hand through at the tip-end of the stretched rubber.  You can put your hand through this, and also clip the float line on with no chance of it sliding back.  You do need sufficient rubber so that you can still load the spear when there is a knot in it.   

Went weka hunting too! That was almost as much fun as diving.