The midplate is 0.065" thick aluminum plate.  It is not known if all comms had the center cut out, but it is our guess they did:

Where to get:
Buy 1/16" (0.063") aluminum plate stock.  You will need to start with about 4.25" x 2.75" per.  This is easy and cheap material to find, available in many hardware / hobby stores or online anywhere, including often eBay.

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