It's very easy to make a P-weight. All you need is:
Disclaimer: Molten lead is incredibly hot and can burn you horribly. Lead is toxic and its fumes are poisonous. If molten lead meets water, it WILL explode and go everywhere. I absolutely do not advise anybody to melt lead, you do so entirely at your own risk.
The basic process is:
Put lead in saucepan, and put saucepan over fire - A camp fire would do, but I used a Coleman's stove. I also used thick gloves and an old undersuit, just to make sure that I was safe from any lead splashes that might happen.
A much more detailed description of the process (which I used when making mine) can be found HERE
I used a rather longer P-weight bolt than is traditional, so that my V-weights could use it too.
Other Mutations:
P-weights are versatile things - some don't even use bolts! Take, for instance, the W-weight, invented by "Wacker" and forwarded to me by Daz of Yorkshire Divers:
The flat pieces of metal go over the bolts your twinset uses, therefore the P-weight needs no bolt of its own - an ideal solution if your backplate has no central hole for a P-weight bolt to go through. The same goes for the modified V-weight (the one at the top).