Cut it out with a slitting wheel or air saw and weld in a patch, or if you're not worried about it being flush a patch welded straight over the top would work fine.
My concern is that the metal is quite thin around that area. I only wish I could get to the hole from underneath. Could I repair it any other way than welding ?
Ignorant newbie chipping in, expecting to be shot down in flames.
But im interested in the answer myself as well.
I cant work out the scale from the photos. It looks quite a small hole, less than a cm?
Is it possible to fill in a small hole like this by clamping some copper on the back and filling/building back up with weld? Then grind back.
Or if its bigger a small disk/plug of steel but still with a copper backing piece and filling/building up the gaps around the plug?
As I understand it the welding wont stick/penetrate the copper but will sit on top and will join with the surrounding steel and plug.
I would guess its nearer 20 x 30mm hole. If you don't want to cut the hole square and patch it properly you could cut a piece of steel and push it up to the hole from below and weld it up then sand it flat.
Looks like a strut top to me. the bolt holes on the actual strut top either size od the big one are max 10mm, prob for an M8 bolt. Should weld up easy.