In reply to climber david: Lots of things to consider, especially for something to be classed as a 'permanent' facility. Rule 9.79 means you need a written operating manual signed off by a Technical Adviser and that will include every detail of what equipment is needed. Although. it does mean it is then fairly easy for people to be subsequently authorised to run it as an activity.
For instance, you need to consider whether you should be using full body harnesses (or sit harness + chest harness) for crate stacking. The Scout Association doesn't give any firm guidance on this, but Girlguiding UK actually mandates it - see
http://guidingmanual.guk.org.uk/activities/a_to_z_of_activities/campfires_t...
Full body harnesses with a rear connection point high up are probably ideal and if it is likely that Cubs will use it, they are a good option. They mean less chance of hitting the floor on rope stretch if falling backwards. Equally, plenty of people use sit harnesses with no great problem. This is the sort of thing you need to get advice on before you make too many plans.
When running crate stacking (one rope) what I'd want would be something like:
2x Full body harnesses (stacker + next person in the queue to put one on)
1x Sit harness with adult belayer (or 2x sit harnesses if Scouts swapping over)
1x Belay device + krab
3x Helmets, for belayer, stacker & crate passer (4x if someone is tailing the rope for the belayer) - everyone else should be kept far enough away not to need them.
1x Ground anchor (sandbag) if Scouts are belaying
What you run your rope through at the top anchor and which method you use to connect to the harness also needs some consideration. There are numerous options some of which are much better than others and you might need specific kit like steel carabiners, maillons, DMM Belaymasters or such like.
There are plenty of options but by rotating participants around you can certainly get away with fewer harnesses and helmets. For example I would far prefer having a proper ground anchor (as used in climbing walls) instead of extra harnesses.
Anyway, there should be advice available about all this at County level (and if failing that, at National level).