Thanks Kirk for the insights.
Don's original vent tube did go into about the center of the mold but was more or less completely blocked with plastic inside. I drilled it out before my first run and after the first run it was mostly blocked again so I'm not real confident that no powder would spill out although I didn't notice any of my "flakes" in the oven. There doesn't seem to be any problem with this particular mold not having a vent.
As far as the tricks go, the mold seems rather well designed with the heat-sink like assembly flanges hopefully causing the walls at this largest diameter to be thicker than the rest and the mass of the plug and clamping plate hopefully adding material to the tip section. So far the tops have been nice enough that I'm reluctant to saw one open to inspect the cross sectional thicknesses of various regions. If they continue to be of good quality, I'll probably not sacrifice one until I have at least one for each of the contributing members unless the balance is particularly bad in one of them. I think my void problem is not a material collection issue due to poor heat distribution but is rather due to the material being completely packed into the mold and initially not being able to roll around and tumble as powder would in a partially filled mold. The lathe swarf chips or flakes kind of bind together and if there is an unfilled spot while it heats up, that spot could remain empty throughout the process because none of the loose material has an opportunity to fall into it.
Here are some pictures of the materials I'm using. The swarf in the bags is just the waste from shaping and hollowing the cast HDPE tops. In the bin is material from the bags selected to fill the mold and then cut up into small bits or flakes to make loading and packing easier.
The bits and flakes are loaded into the mold through a funnel via the opening in the body of the mold where the tip will mount. I use a 1/2 inch dowel to ram in the plastic to maximize the material charge. The mold is filled and packed to the brim and the end plug with screw then gets pressed in.
Here are a couple of the tip mounts where the voids generally appear. The blue one (third from the left) is nice and complete while the others are not unreasonable and should be reparable with a little hot air filler welding.