Hopefully you got those for next to nothing.
Call GPA ( Great Plains Audio ) and ask them if they will sell you a replacement Phase Plug for either a 288-8K or 299 driver.
- If not, you have essentially a fog-horn wannabee on your hands.
Back-caps can be fashioned out of almost anything circular of the proper dimensions >> I wouldn't be surprised if Ikea has a salad//mixing bowl that might be made to work.
Diaphragms?? Unless original GPA's are used, all bets are off >> though the first aftermarkets that I would try would be coming from ZXPC who are located in Florida ( they source Chinese parts for resale and hopefully should care a bit more about their reputation than a direct seller straight out of China >> IMHO ).
ZXPC Altec-style aluminum aftermarket diaphragm
