For a while I had some plastic trays I had picked up at the container store. They looked sort of like this: http://www.containerstore.com/s/collect ... 714C525F59
But, quickly couldn't keep up with the number of heads out of their original drives. So now, I just put the original bad heads in the donor (leaving the spacers in place for easy removal. Then after imaging is complete with donor heads, I re-assemble the client drive and re-assemble / re-test the donor after to see if the heads are still good, and put it in storage for possible future use.
Though if the client heads are mangled, I may just put them aside until the project is done and I'm ready to re-assemble.