You can definitely do it.
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I managed a 20" hardtail in my old mk2 any number of times, without removing any seats, and with a
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(slimline) sub box behind the passenger seat.
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It wasn't even that uncomfortable.
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Both wheels off, seatpost out, frame in with the chainstays down in the passenger footwell and the bars/fork turned to whatever is most comfy, then fit the wheels around that.
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Seatbelt it up to stop it shifting when you're driving.
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You may want to put an old blanket down first, cos I was super annoyed when I ended up with dirty oil all over my seat bolster.
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