Fixing through the ply into the steel (as it was before will make a firmer job than coming up through the steel and into the ply. Also if you did it that way round you would have to make sure the screws don't break through the top surface of the ply. I know you know this
For about 3 quid you can buy a countersink bit, which has a grub screw on the side which tightens with a small Allen key, so you can fit small hss drill bits into it. Get a few hss bits, about 2mm, probably about £1 each, and you can get though the ply and the steel easily, with the holes nicely countersunk. That way the screws will be recessed neatly with no splits in the ply.
I just dropped in, to see what condition my condition was in
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