You should have two brackets that secure the lip on the lower, front edge of the battery. Look for two 6mm studs on the front of the tray.

Most parts stores sell a generic version of those brackets.
http://www.oreillyauto.com/site/c/detail/OSS0/01329.oap?pt=N0363&ppt=C0005

Also:
http://cgi.ebay.com/ebaymotors/2-Battery-Bracket-Mercedes-Benz-230-240D-280E-300CD-TD_W0QQcmdZViewItemQQhashZitem4397e08729QQitemZ290310883113QQptZMotorsQ5fCarQ5fTruckQ5fPartsQ5fAcces sories#ht_776wt_941
The bracket on the right looks like the ones on my '82 300D; both of mine are alike.