Blakes are the best profiles, but awful expensive if they are not being used much. Only worth buying for pricework, better off learning how to build nice corners first.
Alternatives would be a nice straight bit of 4x2, or the box steel they use to fix cable channels above ceilings in factory new builds etc. Build a corner of few courses nice and plumb, then clamp on to the perps with some sliding clamps. Can even mark you gauge with a pencil on 4x2