As it is will be summer you could dive nude, just with a neutrally buoyant cylinder, fins mask and reg, you would not need to carry much kit up the hills or worry about gathering rocks to use as weights. You would probably get lots of publicity too.
For training try running up and down stairs with a stage cyl in a rucsac 300 reps should be about 900 metres of accent and descent.
If I could get time off work I'd come and offer support at Red Tarn.
Good luck with the event
Lake Diver