the reason for the sloping inwards top, is to channel/guide/push the birds to the ladder area, having a flat top, they could simple sit there, without going near the ladder, ladder spacing is sometimes trial and error, so make it simple and adjustable, make sure the ends of the ladder are solid and not open, otherwise birds can climb the sides and get out
cant see any reason not to make it from things to hand, ie, wire fencing, as long as the shine dont put birds off, may need spraying?