I’d like to make it automatically stretch when the arms move:
These hands are a part of a rigged character, so they don’t have their own origins. How can I achieve this? Supposedly there are multiple ways to implement this, but I can’t figure out even one.
Use stretch to constraint. Create three bones chain middle bone should match length of your box you want to deform. Parent bone of that bone should be parented to bone driving one of the hands, Third bone should be parented to bone driving other hand. Now select this third bone, shift+select middle bone and (ctrl+shift+c) add stretch to constraint. Then add armature modifier to your box and apply weight to middle bone. Your mesh will be stretched and shinked between hands as you will move hand bones. Lol it would be way easier to show than describe. If you don’t get idea i could make some simple example blend.