Here's how I made mine:
2 circles of 7-8 Oz leather (er... 3-3.5mm?), diameter as applicable. (around 10" was not a bad size for general silver work, engraving and the like). Stamp name on before assembly!
I glued up the perimeter 1/2" or so, leaving a gap of about 1 1/2" for filling. 2 parallel lines of stitching (mine were grooved first with a compass race, nice but not all that neccessary) about 5-6mm apart, probably 6mm in from the edge, staggered start (probably not needed, but good practice I felt); leave the gap unstitched for now. Case the leather, bag it & pop it in the fridge overnight; remove the following day and fill. I used silver sand from B&Q - BUT (and this bit will make life much easier) - dry it in the oven first. Trying to stuff these with damp sand is a PITA... Tuck a funnel into the gap between the discs, pour the sand in and pack it in as densely as you want (less densely packed makes for a good surface for forming metals into). A stick can be handy to poke in the sand when it sticks in the funnel. Finally, finish the stitching, trim the threads and let it all dry out properly; I gave mine a very light coat of neatsfoot oil which seems OK, but others work just as well without the oil (and perhaps some of the metal chips don't catch as much on the surface as a result).
HTH!