This is an absolute, bare bones hack. I don’t recommend it under usual circumstances. Work very slowly. Pay attention.

Get 10 large heavy rubber bands, heat some water, put the large rubber bands around your violin body, using a brush, apply the water sparingly to the center seam, using small round wood pieces, like dowels, slide them under each rubber band and slowly put one twist in each rubber band and secure them under the band. Protect your varnish with pads of fabric under the dowels. You are creating a rudimentary vice. As the crack closes, add a little more hot water, put another twist in the band with the dowel. Go very slowly. Not too much pressure with each movement. If the crack has closed under one set of rubber bands, don’t put a twist in it.

The problem with using this solution is that the top or bottom plate may come loose from the sides. If that happens, you can use hot water on a brush in the area that has come loose, and under the pressure of the rubber bands, it should rejoin.

This advice is based on my own experiences, with a strong disclaimer that you should always seek the help of a luthier when problems like this arise.