Foot rest device for older child's carseat/booster?

[PrettyCandles]

7yo ds is getting pins-and-needles in his feet when we go on a car journey. Any idea whether there exists some sort of foot rest or sling that can hook onto the car seat to support his feet so that they won't go to sleep?

cardboard box in the footwell?

We need something that won't obstruct the footwell, as I tend to load and unload all the LOs from one side. Also, I don't like loose things - I've been in a relatively minor accident, and the way things flew about the car was shocking.

But I've found this.