A weighing dish, sometimes called a weighing boat, is often used on a scale when weighing small quantities of modernist ingredients. They come in a variety of shapes and sizes and are typically made of polystyrene or aluminum.
They can be used for both powders and liquids. They normally have slanted sides or a pouring spout to help to control the transfer of the ingredients out of the dish.
An inexpensive alternative to a weighing dish is just a small slip of paper placed on the scale. This method is only practical for relatively small amounts of ingredients since you don't want to lose any powder as you transfer it from the scale to the destination.
When using a weighing dish be sure to "Tare" or "Zero" the scale after placing the empty weighing dish on the scale.
If you feel you will use a lot of these you can get them inexpensively on Amazon in a variety of sizes. The Plastic Square Weigh Boats Small Dish 100pk holds 20 ml. They also carry 100 ml and 330 ml boats (all shown above).
Modernist Pantry has three sizes available as well. The Large Anti-static Weighing Boat holds 270 ml and has the "canoe" appearance. These dishes are very high quality and ideal for weighing quantities of static-affected materials, both powders and liquids. The canoe shape enables easy pouring and transfers of weighed samples.
Made of polystyrene these dishes are contaminant-free and biologically inert, resists dilute acids, aqueous solutions, alcohols and base. They are also able to withstand conventional heating to 158F (70C).I found one of those promotional cubes of 3" x 3" sheets of paper I was given by a vender which is perfect when I just need a sheet of paper on the scale. You can buy "official" Weighing Paper, 3 x 3", Pack of 500 if you're so inclined.