Defense Distributed bills their Serial Covers as “The solution to maintaining privacy while still looking cool.”
You get three segments of 3M-backed self-adhesive Cordura material to prevent your serial numbers from being easily visible and traceable, particularly when posting photos online. Offered in multiple colors amd patterns, the shapes were
configured around a Mil-Spec M4 lower receiver.
Available from defensedistributors.com but of course, you could do this yourself. However, if you buy them Defense Distributors you support their development of other products.