Here's an example of how to create custom weights:
Find the density of your material:
Plexiglass = 1,18 g/cm3
Determine how many cubic cm are required for the weight you need:
volume = desired weight / density = 10 g / 1,18 = 8,475 cm3
Divide this by the thickness of available material, to obtain the area to cut:
area = volume / thickness = 8,475 cm3 / 0,5 cm = 16,949 cm2
You can now cut any shape that has an area of 16,949 cm2 and it will weight 10 grams!
It is possible glue pieces together to make other shapes, like cubes.