Yup! And you can melt the plastic in layers so you get a nice solid core, or fill the very center with rocks or weights or what have you to reduce how much plastic is in it.

I would recommend getting some heat prof material to set the mold on, in, and some heat proof gloves as well in case you need to use a small metal bar to move the melting plastic around

Bonus points for separating your plastic into colors, lets you do some cool swirly affects and what not vs. just a mishmash of colored specks