I experimented with the Tunisian reverse stitch for a 7×9 block for Warm Up America. In Tunisian simple stitch you pick up the front vertical bar, in the reverse stitch you pick from the back of work and the back vertical bar. It makes a very thick pleasantly squishy fabric.

Blocking the rectangle will probably even out the edges and reduce the slight curl. The tutorials I found claimed this is a flat stitch, but my sample has a vertical curl, not nearly as pronounced as the horizontal curl of simple Tunisian crochet, but still present. It could be my edge stitch tension or technique, though. Experiments will continue.