I havent done this in a very longtime. I used to pickle prawn. I would even make small containers for small trips. Use sea water, and then ad lots of salt. The salt content must be very high, the water will barely freeze. Normally just around the sides of container and maybe the top. The prawns would be as firm as the day they were pumped and even the same colour.
Not sure if it will work for you, but worth a try. I kept prawn like this for a few weeks.