Suppose that a Green Lantern agent wants to assign a friend of his as his/her deputy for certain reason: for example, ensuring the safety of his home world while he/she is fighting crime several light years away. Could he/she use his/her willpower to create a copy of his/her ring?
Granted, the process might require a huge amount of power (from the ring and from its user), perhaps too much for it to be practical (conservation of energy), but could you make, at the very least, an imperfect copy in theory?
Just to get extraneous issues out of the way, let's assume this friend of agent is strong-willed enough to be a ring-bearer (perhaps even a prospective Lantern!), and the Guardians have sanctioned this experiment for the time being (out of sympathy or at least scientific curiosity).
Furthermore, I believe Jordan's Green power ring has been 'copied' in at least one comic story-line (Superman: Red Son), though I haven't really read it and don't quite know how that universe's Jordan accomplished it.