It sounds like you are after something large, so I would echo what others have said about using sheet materials or one of the popular cut and check techniques.
However, for an engineers square I can recommend Kinex. I recently bought a couple of squares from Workshop Heaven after originally looking for some Moore and Wright. I hadn’t heard of Kinex before but they were cheaper and advertised a higher standard(DIN 875/0 a German standard). Lower cost higher accuracy didn’t make sense but for the price I didn’t mind risking it. Very happy with them after checking them, and for £30 for 150mm I’d not be too shy about using it as an everyday square!
EDIT: Just to say that I was comparing Moore and Wright at BS939/B to Kinex at DIN875/0.
I wasn’t suggesting that Moore and Wright don’t make squares with a higher standard.