If I could find a child seat that retains the 5 point feature at higher heights/weights, that would solve the problem. When they are in this configuration a lap type belt is all that's required to hold the child seat in place. The child seat then has the 5 point harness built into it. The problem is after kids reach a certain height/weight, these child seats are meant to convert into a booster seat. The 5 point harness is removed and now relies on the standard shoulder/lap combo belt to hold everything (child and booster) in place...Novas unfortunately lacking the rear shoulder belt.
Totally understand your point and I know this doesn't solve the original desire of the added shoulder strap as far as the child's restraint, but for the booster seat itself i feel like these are much more secure.
Here's pictures of my youngest sons booster with the clips I was talking about and the "hooks" in between the cushions I was referring to. With the car seats and boosters I've used over the years there has always been slack in the seat itself when only belted in, i remember one time my sons seat (when he was like 3) actually tipped over around a corner and it was belted in. Since then I only use the hooks, clip them on and pull the straps and they don't budge.
But obviously this is only the seat itself and not strapping the child in. Just a thought, either way if you wouldn't mind giving a update if you go through with something on this as I would like to see the outcome!
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