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Udi Tirosh, of DIYPhotography, sent us the link to this post on creating back-to-back ring adapters for macro photography on the cheap:

One method of delving into macro photography is to use adapter rings.

These rings have threads on both sides, one end is screwed onto your present camera lens like any filter and a second lens is attached in reverse onto the other end of the ring. So two lenses can be attached front to front using the adapter rings filter threads. You can buy these adapters or you can make them yourself by taking filters matching the thread sizes of the lenses being used and removing the glass and epoxying the rings together with the threads sticking out on both ends.

Macro Photography On A Budget Or Back To Back Couplers