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Gate frame

Hi. I'm after a frame for my gate. I want to attach pailings to it but thought a metal frame would be strongest and least likely to warp. I need a simple metal frame out of tube or channel that I can drill into to attach the pailings and latch etc. Dimensions are: 1600 one side x1125 wide x 1300 the other side (so a slope on the top) and a diagonal for strength. Also hinges 150 from the top and bottom on the 1600 side. Measurements are in mm. I need two of these (one for each side). Pailings to go on road side and gates to open outwards. It will need to be rustproof, open to suggestions as to best option for this, I.e. galvanized, aluminium, painted? There will be a lot of holes in it so not sure what the best option would be. Thank you.