Simplest things are always in fron of us.
Make spliced adjustable loop, tie standing end of with stopper knot (in this case I have actually used the original stopper knot, very decorative.) On working end tie of with another stopper knot (sink stopper this time, nice big knot for toggles). poke working end through adjustable loop to close.
Thanks to Nacra for starting this wonderful thread!!! It has really got me thinking.
The example here is tied in some old 3mm guyline I had knocking about, should work fine in amsteel though.
Now all I need to do is find some Hollow braid Dyneema in the UK, everything seems to be over a core here!!