Gear lights (aka gear warning system) are directly related to AD 46-04-01 which called for regular inspections of the factorys mechanical throttle-lock lever interconnect cable.
A quick description: The factory rigged a shelded, flex cable between the carburetor throttle arm and the gear locking lever such that when the gear pins were not locked (lever in "raise" or "lower") the throttle was blocked from retarding below about 1700rpm in flight. The idea was that if the engine would not retard below a slow-flight setting the pilot would remember to lower the gear so as to get idle rpm. All was OK until someone panicked and pulled on the throttle hard enough to bend the blocking pin. This restricted the gear locking lever so that he was unable to lock the gear pins in place sooooo, he geared-up.
Almost all of these old flex cables and stop plates have long since been removed and you will find a wide range of "alternate" gear warning setups... very few having any kind of approval. If you have a Cadet that has no electrical system you have no choice but to keep the factory system working and rigged correctly.
In an effort to "do it right" on my Cadet (N37814) I went to the (long and complicated) effort to get an AMOC (Alternate Method of Compliance) for AD 46-04-01 which was a "remove and replace" with lights, switches and a horn. This required drawings, parts lists and instructions for continued airworthiness and the approval came in the form of a letter authorizing the same installation in other Cadets as long as I (the applicant) did the installation. This is NOT an STC but it IS approved data for other installations.
I have made no effort to do other installations. I have never even tried to calculate a reasonable pricing... To do so, parts would have to be machined / ordered, paperwork would have to be generated and (per the FAA letter) I would have to do or supervise and sign off each installation.
If there is enough interest in this system I may consider puting together some kits and helping you get your gear warning system in compliance. Let me know what you think.