There are many ways to store and transport kwads, from special soft-bags to strapping to outside of backpacks. I'm a fan of hard cases and tool boxes as they protect during both transport and storage. Since I've been enjoying 3d printing projects of lately and have a cheap foam block case to experiment with, I removed all foam blocks from it and designed a simple fully parametrized mounting system for quads.

It relies on a base plate with two wedge shaped arms to slide the quad into, and a rear locking mechanism to prevent it from sliding out. The wedge arms are printed on the side, to provide a very smooth surface to not catch on quad arms, and are glued into base recesses. The locking arm is profiled to have flexibility allowing for easy snapping onto the rear edge of quad's base plate. Both the base plate and the locking arms are attached to the case with rivets, but bolts would also work. Since my FreeCAD model is parametrized, it can be scaled for a bigger quads, by making appropriate measurement changes with FreeCAD recomputing the shape automatically.

I like to fly 2-3" micro-quads and this case can easily support 4-6 units of that size, but can also be easily setup for 2x 5" (without props), or 1x 5" (no props) + 2 micros.