I think this is the best option
Guns Modify SAI RMR slide prototype for Marui Glock 17 Glock 18 Pre order now ! - YouTube
Recently discovered is the SAI RMR slide on a glock 18 has a hidden fire selector that can be set on semi. That would "covert" your glock18c into a glock17 (losing the external fire selector)
Short of buying a whole new glock, i think this is the cheapest option available.
-alex