This may to thick to go under the back seat, but give it a try. Another place may be suspending it under the package tray in the trunk.
I have quite a bit of electronics under the rear seat of my 123 car. Keep in mind that if you carry passengers in the rear seat very often, you may get a question from them asking what they are sitting on.
'84 Euro 240D, 516K miles
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'81 300D Daughter's Car
Over 800,000 miles in