The Chillybuddy guy had changed the design slightly when last I emailed him, some years ago.  They make custom sizes for $15 extra, which you might consider.  My Medium is just barely long enough -- he says you want it to cover their butt -- he changed the belly band to have velcro on both ends, to better modulate the fit.  A corgi is like a Small girth with Medium length.  Pay attention to their specs.

I carry ours in a heavy-duty plastic bag.  Add water to the bag to wet it (must be damp for evaporative cooling).  I usually carry a pint plastic bottle to save dog-water leftovers when water is scarce -- can't always afford to waste the water they don't drink.  When strands of mylar pull loose, just cut them.

We do not have thorny brush here; stuff like blackberries might be trouble for the Chillybuddy.

Do teach the Corgi Toss and encourage it.  They WILL try to do dangerous stuff like trying to leap logs 3' high, and I hypothesize that Al got hurt on a backslide from something like that, getting his leg caught in the tangle.

And they DO get these CCL injuries, which never heal perfectly.

You might check out some of the hi-tech harnesses with a handle on the top.  They are unnecessarily heavy (I made my own lightweight version).  Once you get it sized, you can cut out the adjustable buckles and stitch it permanently if you're good with a sewing machine.

Whoops, I see you already know of John, Gwynnie and Al :)

Welcome Rebecca and Timber! Look u0p John Wolff, he and his corgis hike all over Oregon. My SIdney is a therapy dog :)