Tying with thigh cuffs
Buckle a thigh cuff high on each thigh where it cannot slide, then clip each wrist down to the one beside it.
How it is done
Thigh cuffs solve exactly the join that started all of this — wrist to thigh — with no rope at all. Buckle them high enough on the thigh that they cannot work their way down over the knee, then bring each wrist to the cuff on that same side and clip. Because both ends are buckled hardware, the join stays the size you set it, which a rope join does not.
What goes wrong with this one
A thigh cuff that slips down is worse than none: it ends up holding a knee joint instead of a thigh, and the wrist goes down with it.