they have other thicknesses also, but i think 8mm seems a good balance of strength and weight?
you would use a hole saw of both inner and outer diameter, creating 2 pieces, then welding them together, and finally weld that to the mallet head, so it would fit inside for strength
http://www.directplasticsonline.co.uk/HDPESheet/Natural/8mm/
they have other thicknesses also, but i think 8mm seems a good balance of strength and weight?
you would use a hole saw of both inner and outer diameter, creating 2 pieces, then welding them together, and finally weld that to the mallet head, so it would fit inside for strength