Cheers youme, I'd just come back to say that I'd found out how to do it here, which was also written by you, so thanks for the help twice

Youme is right,
1. Set up a trigger_multiple that kills / spawns a new box.
2. Make a new entity, "filter_activator_name" and set these parameters
Name - box_filter
Filter Mode - Allow Entities that Match Criteria
Filter Name - box_name
3. Set your trigger_multiple's "Filter Name" parameter to box_filter
4. Make sure your box spawner spawns a box with the name of box_name.

And that should do it.

I just ammended this info to the wiki

Now, how do I set this up if I want to have a box already positioned on a ledge, then the box dropper drop a new box when the original box is destroyed, just a back up incase the player is an idiot and drops the box in the toxic goo?

Also, weird question on following that tutorial.
for some reason mine box dropper lid opens twice on the first spawn
and only the first spawn.
It'll open to drop and then immediately open a second time.

Any ideas? I also tried "Not removing template items" as a flag
as well as the preserve entity names just in case, but that's as far
as I can guess.

that may be a timing issue. I'm sorta following what the box dropper in test chamber 5 is like, where there's a trigger that closes the lid after an entity has stopped touching it, and is opened by another trigger.
Of course, I have no idea if it works, because nothing else is.

I might have this working, but I've been saying that all night, so I'll report my findings when I get it working, or have more questions, which will likely be to do with filters...

Well that was an evening well spent....
I've got it so that it doesnt spawn the new box until the first one is destroyed by putting a func_nobrush keeping the template box from falling down into the visible box dropper. the func_nobrush disappears when killed by a trigger once.
Once the box falls, it hits a trigger_multiple, which opens the box lid after a delay, then is supposed to close it once the box has fallen out and cleared the area.
But, the lid is closing on the box too soon! Its like what Yekyaa was saying, it sorta twitches open and closed.... It seems to play the open animation, but close again right away. I might be able to discern some info from the official maps tomorrow, but I need some sleep for my final tomorrow morning. So anybody have any ideas?

Is there some way to stop the spawning mechanism to stop spawning a box at every box located at the map, without making an individual name for each box, and therefor also a filter for buttons and such, for each?
It's quite unconvenient if a map has more than one spawner or other boxes lying around, because if you loose your first box a couple of times, and there suddenly is a pile of boxes where there were one.