Tungsten weight
You can purchase Tungsten in flat bar or rod from McMaster Carr.
It’s very expensive! And very hard … usually 65 Rockwell and nearly impossible to cut or drill into unless you have the proper tools and equipment. I suppose you could use it for an all day stretcher weight depending on how you plan to attach it to your penis…. good luck!
3/4” diameter rod x 6” long - McMaster Carr p/n 8788A259 - $141.80 + shipping
1” diameter rod x 6” long - McMaster Carr p/n 8788A263 - $294.74 + shipping