Custom cardboard boxes are cheap enough, you will pay however for the design of the cutting dies, and also they tend to have a minimum number of units you have to order. Can you not see if a manufacturer of cardboard boxes has a ready made version that will suit you, and then simply make a cheaper cut board to have the knob supported, which is a lot cheaper to do, and your box at least will be off the shelf size wise. Pretty much all corrugated carton manufacturers will do so, and try your local suppliers first, then look at neighbouring countries, Poland will probably be cheaper than Denmark because of lower wages, so try there as well, and shipping will be easier for this, as cardboard shipping is quite expensive, as it is done using volumetric mass.
Otherwise a regular box and a die cut foam insert, either in polystyrene, or other foams if you need more flex, or a custom cut urethane foam for fragile parts, but those are both more expensive and less environmentally friendly as they are typically harder to recycle, unlike cardboard.