An old Iai trick is to take a bamboo chopstick - the cheap ones from the supermarket are fine, just make sure it's bamboo not pine, and cut it to fit. 5 minute job and your done, I wear one out about every 2 years!
using metal is said to be ok if you chose your metal, old tachi had Gold mekugi, the soft metal is said to absorb stresses. The mistake is in thinking you need someing like steel on steel - just a bit of common sense really.