How snug is the right snugness... well, the Morse concertina bellows are made on a jig such that their "natural" state of rest is partway open. When we finish making a bellows, we take it off the jig, exercise it for a while, and then compress it under heavy metal weights for at least 24 hours, and then exercise the bellows for a while again prior to assembly. A newly-built Morse will tend to expand a bit if set on a table, maybe an inch or so. So when putting the corner blocks in a clamshell case, my goal was to size them so that when I compressed the bellows of the instrument just so it's closed I could easily set the instrument into the case, and when I let go it wouldn't expand. But I didn't want to have to compress the bellows with any force to get it into the case; I just wanted to compress them closed, not force them closed. It was a "you know it when you feel it" sort of thing.
My main squeeze is a Jeffries with a Dipper bellows, and the natural resting state is closed, but lacking a nice jug case, I use a soft case whose chief virtue is that anyone not in the know would think it contains my lunch (and whose secondary virtue is a front pocket just large enough for a small toolkit and a Zoom recorder). I was initially using this soft case for a Morse, so rather than putting corner blocks in, I just hot-glued the center of a strip of velcro to the bottom of the case, so I could compress and buckle my concertina into the case like it had a seat-belt. This has the added bonus that, if I pick the case up by its strap having forgotten to zip the top shut, the instrument doesn't fall out because it's buckled in!
When I eventually make a jug case, I can imagine a different use for velcro: assisting in getting the instrument out. If the velcro is looped around the instrument, and there's a bit of a handle, then you're pulling on the velcro (which is going all the way around the instrument and pulling the whole thing) and not just pulling on the near end's handstrap (against the vacuum of the bellows). This might be overkill if the jug case is designed just perfectly for the concertina (such that the instrument just pours out of the case without need for pulling), but I doubt my case-making skills will be that perfect!