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KFKiter said...
Drill holes either side of the stringer, around 5mm, pick out a decent amount of the foam. leave the surface glass structure as intact as possible, fill up the holes with a liquid epoxy. I used Megapoxy HX, it is almost like water and penetrates into the surrounding foam. Spray you screws with a release agent and push the screws into the holes. the screw heads should be wider than the holes. leave for two days undo the screws, screw foot straps on. if you want to make sure of the strength lay up a piece of fibreglass over the new screw holes, screw your screws back in and leave for another day. undo the screws and let the epoxy cure for a couple of more days before tightening your footstraps up too tight.
As an addition to the above suggestion you could use a trick that boaties use for attachments to cored decks. To remove the foam under the surface glass you can use a
small "L" shaped allen key in a drill.
put the long part of the key into the drill chuck.
carefully insert the key through the small hole in the deck, then use the drill/allen key combo to "mince" the foam between the upper & lower skins.
The diameter of removed foam core plug should be 2 x the length of the "L" on the end of the allen key.
Care should be taken with the small hole in the deck, so as not to chew it out too much, care also with the depth of the removed plug.
Practise on some scrap foam if you are uncertain about the outcome