I don't know about battery covers, but I bought a 707 tempo knob from these guys:
http://cgi.ebay.com/ws/eBayISAPI.dll ViewItem&item=300182051545
They have alot of parts for vintage gear manufatured new in China.

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To: tr-707@yahoogroups.com
From: jeffjary@...
Date: Mon, 28 Jan 2008 13:05:04 +0000
Subject: [tr-707] tempo knob and battery cover

hi all! i'm looking for a tempo knob and a battery cover.

thanks!
jeff