Why not screw or rivet some galvanised angle to the cappings. Then you just need some cross bracing on your ply to make it stiff enough.
Printable View
Something like this may do the job.
GN 113.5 Ball lock pins, Stainless Steel 1.4305 | OTTO GANTER
They may not be cheap though.
If you want some pins turned up or some bolts drilled give us a yell.
Cheers,
Mick.