We had exactly this problem and also have the same MH.
We solved it by putting a luggage type ratchet strap around the supports to the headrest, and then around the extension to the head support on the booster seat back.
Hope this makes sense.
It does. I tried similar, but wasn't too happy with the base wanting to move.
I think I may look at welding in the Isofix loops or have the seat foam replaced with an added bolster on the edge.