Traditional malts and milkshakes are made using as metal cup and wand-style blender. The ingredients, milk, ice cream and flavor syrup, are added to the cup which is then mixed together by the spinning blender wand. Either the malt cup itself or a separate glass are then brought to the customer for their enjoyment. The only major considerations when selecting a malt cup is the capacity, height and whether or not it has a handle. The height of the cup will let you know fit here is enough clearance for it to easily fit under the blender wand. Capacity dictates the portion size. You will want the cup to be filled completely full after everything is blended together, so the total capacity will indicate how large of a serving glass you will need.
However, some establishments simply take the metal cup to directly to the customer. This further enhances the handmade quality. Other establishments that want to serve it in a different glass often bring the malt cup to the customer anyway, because the serving glass usually holds less than the mixing cup. If you never intend to bring the cup to the customers, you should consider on with a plastic handle for easier gripping.