To make a nail knot in the field, using a nail or a needle is definetely old-fashioned and difficult!
A much better option is to cut a 2-3cm (about one inch) piece from an Earwig (or cotton buds, whatever you call them), so it forms a small plastic tube.
Then simply tie the nail knot so that the tube is inside the knot, thread in the end of the leader and remove the tube and tighten.
It's very easy to tie this knot even with cold and shaking hands.
I put the plastic tubes in every pocket of my fly vest and wading jacket, so they are always available. So far the nail-knots have not failed once, even when fishing salmon or big pike. The ready-made loops however, have failed twice.
Samuli from Finland