I suspect you would have to find a sheet metal place that would fab one for you. Out of curiosity (a) why do you need an extra fuel tank and (b) wouldn't your spare tire then "take up all your trunk space?"

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"Buster" in the '95

Our all-Diesel family
1996 E300D (W210) . .338,000 miles Wife's car
2005 E320 CDI . . 113,000 miles My car
Santa Rosa population 176,762 (2022)
Total. . . . . . . . . . . . 627,762
"Oh lord won't you buy me a Mercedes Benz."
-- Janis Joplin, October 1, 1970