I would love to see a photo of one of your final melted blocks if you should turn another. Do you think it would be possible, or helpful even, to pound the melted plastic mass as one might do to cake mix to prevent air bubbles/voids or is the plastic not liquid enough?
Here's a pic with a container full of raw material chunks which gets melted and additional chunks added several (maybe 6) times next to a cooled 'fully' combined block of HDPE plastic in a similar container. You can see the surface is continuous and smooth but not a nice uniform shape. The shrinkage is reasonably visible. The inner diameter of the container is about 4 inches and while at temperature (360 F) the material fully fills the diameter. The shrinkage is so nonuniform around the circumference of the resulting cylinder that after cleaning up the outer diameter, I get a suitably round turning blank to make a top of 3 inches diameter, which is the nominal size of the white top in the size comparison picture posted earlier.
Not going to high enough temperature will probably result in a very skeletal block unless some real pressure/compressing force is used. Here's my very first test of the mold with milk jug plastic. Of course I underloaded the base form (due to fear of terrible adhesion) but regardless, the material didn't flow together under gravity and it really didn't mash together very well when the inner part of the mold was pressed in.
Pounding may do something, but it probably will be rather difficult. Even when hot, the material is very 'stiff' and when removed from the heat, the surface quickly cools and solidifies thus making it hard to work. On the top of the block you can see neat tight 'swirls' of red/white/blue which are due to my stabbing a rod into the melt several time in the hope of venting and collapsing any large voids--don't know if it worked yet, may have created more. I think stabbing is still a good idea because it provides a test of the melt in the center. The material melts from the outside to the inside so the surface can look very nice while the chunks in the center are barely warm and therefore still just individual chunks. I think with a little experimenting you can not only wish for reasonable uniformity but you can expect it, but don't be disappointed when they aren't perfect.
Maybe Neff will be making his own custom discs for golf.