Spreading the load
When you use a towhook you're placing a several-kilo load on a single point. Whilst this is fine on a balsa or fibre-glass plane with a hard point to take and spread the load, a foamie will normaly tear (or fail in some other interesting manner) under the load. I've used this design on my funstart, mugi and zagi's without problems.

storecards
They way I spread the load from a towhook is to use a store loyalty card (or phonecard). Basicaly you securely mount the towhook to the card and then tape (and or glue) the card to the plane. This spreads the load from the area of a towhook (a couple of square cm) to the area of the card (50 square cm). Using this method you can easily mount the towhook to almost any plane in seconds and if you carry a spare card/hook plate you can replace broken hooks whilst out and about. I've stressed these to the point where the hook has been pulled to bits before the card has let go, so I know it's pretty strong.