I have used a variety of unlikley objects as moulds in the past, a visit to the local scrapyard can be very beneficial, aluminium blocks can be drilled out with suitable drills,and make very usable moulds after being cutin half.... dead cow moulds can be made from an aluminium or steel tube, flattend, cut in half and set in a piece of timber, any length or weight you like, finally and the easiest is to use plain builders sand, fill a shallowish tray with sand, push the end of a broom handle or similar into the sand, you can use almost anything with the same profile as the sinker you want, then, and this is IMPORTANT... let the sand dry, and pour in the hot lead, after you have pushed the wire loop into the bottom of the mould first..
I have been making leads for over 30 years, and have only bought specialist moulds that were a bit difficult to make myself.The fish dont care what shape the sinker is, or at least they have not complained yet.