Good question. I tested heaps. The winning design by a landslide was a metal plate reflector, with essentially two diamonds around 15mm in front of it, side by side, apx 15mm on the edge.

I thought it was called a 'butterfly antenna', but web searching now I don't think that's correct.

I built one for a friend just with wire and pliers. The tolerances mustn't have been that important because it worked well.

Maybe someone on here knows the name of that antenna design?