here is some food for thought ... ... .
In incorporating the ports in the cap, you really need some way of clocking their location. With a thread, that will take additional tooling. I would have to actually thread the part into a fixture before drilling and tapping. That all goes out the window if the mating part is not is not threaded in the same manner and than it becomes impossible to predict the port location.
The quick and easy fix is to put the ports in the center. But, you have limited space available with out putting a hex drive so large that most would have to buy a socket to remove it.
BGiles threw out the idea of eliminating the hex drive and adding the profile of a fluted oil filter. This would make is easier for port location and allow my oil filter socket to perform double duty as well. The down side would be that the owner of this creation would have to have my oil filter socket as well.
Waddyall think?