A reader sent me his solution for a cheap lens cap replacement for his Nikkor 14mm f/2.8 lens (see the original lens cap on eBay).
The QC-103 Fernco 3-in. flexible pipe cap pictured above is a perfect fit and protects the lens front better than any other cap or cover. Fits snugly, and it only costs a few dollars:
Several different sizes and colors of Fernco Qwik Caps may work as lens caps for various lenses (check their Amazon store).