I have bought batteries with adapters. The adapters were spacers that clipped on to the bottom of the battery. See if the battery dealer has one. Perhaps a piece of 1/2" wood can shim the battery up. That is, if the bottom holddown would still work.
After going through a few replacement battery fitment problems, I've since paid careful attention to the size of the replacement battery. This is also because I understand that one should buy the largest battery that can fit in the tray. A battery larger than original should last considerably longer than the original battery.
95 E320 Cabriolet, 131K