Shorter length pads can be purchased, or longer pads can be cut, to reduce the amount of bulk and weight (head to knee length is sufficient).
Self-inflating pads are not good for Scouts because they can be punctured.
A couple of large garbage bags may be suitable for summer hiking in the Sierras, but if multiple rainy days are anticipated a dedicated pack cover is better.
Some backpacks are waterproof, but like a tent, the seams need to be sealed.
Kits are typically “personalized”, but all kits should include moleskin (for blisters), several adhesive bandages of various sizes, a few gauze pads, adhesive tape, and disinfecting ointment.