In reply to si_pritch: not a great deal you can do if you don't want to engrave them yourself. You're probably best adding one of the labelled zip ties to each and then just keeping a good track on usage.
In reply to si_pritch:
cheap engraving tool, or do it yourself with a drillbit etc.
you could do it with a soldering iron or hot nail, might mess the baseplate up a bit/be rough.
In reply to si_pritch: I have just Googled the web for plastic engraving. There are lots of companies which can do what you want. Of course it is a case of whether you can afford it. Personally, I would be inclined to get them done as they can be expensive to replace on a regular basis - unless you include the price of one in your course pricing system.