I have just developed confined space training for in house training for our offshore workers. The course is 1 day classroom and 1 day practical in the use emergency escape sets BA and rescue equipment. I'm not really sure you could achieve much in 30 mins. As to whether this constitutes a confined space the CS regs state that a CS has two defining features 1) substantially (though not always entirely) enclosed and 2) reasonably foreseeable risk of serious injury from hazardous substances or conditions within the space or nearby. I think your training needs to be around working safely in the excavation taking into account things like risk of collapse, preventing falls into the excavation, vehicle movements, personnel access and egress, risk of flooding etc. It really depends on the hazards and in my view what's more important than whether or not this is a confined space is properly identifying hazards and controlling risk then training employees around working safely in the excavation. I don't think any off the shelf presentation or elearning course could adequately address the challenges of this type of work and think something more bespoke is required. I would go back to the client and ask them what they think this awareness training should cover and use that as a starting point.