Mountainbike hose/tyres are prefered since you will probably need 3-4 kilos at least.

Wash the inside!

Fill with led ("shotgungrains")

Seal with glue or cable ties.

Use rucksack buckles

Benny, Jome, Subaquaticus, Octopus, Sebastian

The kinds you can buy. Usually for weighttraining (ancles/wrists). Filled with "ledmarbles"
To the right an idea to put the weight above the lungs - where it should be.
Rules demand a quick release. This type with buckles attached to webbing - within there are cableties
To the left the tube is sealed with glue and then wellcrow has been stitched on.