As I noted in a comment on my previous post, this idea doesn't protect your paintings terribly well from unruly vegetation, but it all depends on where you are taking the canvases. It is quite handy if you were planning on making two paintings on one expedition - and, of course, you don't need to go to the trouble of making a bespoke box for each size of canvas. You just need two that are the same height.
The two solutions that I have come across are clips and clamps. Here they are in action:
I have actually used the clips twice; once when I painted Watership Down and Cottington's Hill, and once when I made a single painting of Cannon Heath Down (I brought an unused canvas home with me - it got a few smears of paint on it around the edges, but that didn't matter).
I haven't used the clamp at all. Part of the reason for this is the lack of expeditionary opportunity since I got the clamp set. But part of it is that the clamp actually has rather sharp points, and I'm a bit concerned about the indentations it makes on the canvas surface (theoretically, they shouldn't matter because they are in the sacrificial edge area that will be covered by the frame, but I worry about holes going all the way through).
It's worth practising with unpainted canvases first.