Another possibility would be to bring the PD by a beacon, like reinforcements. civ-ally-SCU with fabricator upgrade building the pd for you on the desired spot. The SCU could be taken out midway through transporting, and it would be hard to build the beacon in someone's base anyways (if the beacon is destroyed before the SCU lands, the 'call-in' is cancelled. After building is canceled the SCU recalls. HP of the SCU could be also adjusted(?)
This way PDs can be built for free in designed spots, but prevented from being built to far in, as the ACU would have to be present to put the PD 'beacon'.