Necessity is the mother of invention, but I will contend that the common-sense with necessity is the father of innovation. Examples are replete across the industry with various non-firearms materials being pressed into service. Perhaps the most common is the usage of paracord across handguards, grips, stocks, etc. etc.
These field-expedient solutions are often then morphed into physical products. Examples include buttstock mag pouches on the M16, the Magpul, paracord slings, and in relatively recent memory, the usage of grips as storage units. For example, Magpul’s MOE solution is well-known and respected for its modularity and simplicity.
However, one could contend that the commercial solutions combined with the spirit of field expedient solutions produce the best option, like the recent Tac Tip from Tactical Response. In his recent video, James Yeager combines the ubiquitous plastic baggie with the standard MOE cap to solve a few problems at once. Specifically, the plastic baggie allows one to cart around more items than the various purpose-built inserts would allow, reduce the rattle and make the options inside waterproof (assuming a good baggie).
I have since now thrown my firing pin and bolt insert out into the world and replaced that $15 accessory with a 15 cent baggie. Wish I would have thought of that prior to the purchase!