this is what you need
https://www.metalmaniauk.com/Steel/Equal-Angle/Equal-angle-steel-40mm-x-40mm-x-3mm.aspx
Use a 4mm metal drill bit for the holes in the top but also use a metal cutting countersink after. if you can’t get hold of one of those use a 10/12mm metal drill bit to countersink the holes
Then screw down with some 4x40mm screws
That will sort you out well.