I just fished a ryoby Y-branch manifold out of my scrap box. With my Ryobis long gone, you can have it for the shipping cost.
Crankcase diameter needed is 56mm. The manifold seals to the case with an O-ring. Carb bore is 17mm, each branch has 10mm bore.
Two reeds are needed. It now is fitted like the picture, with just one reed.