On my Mk 8 (
not Custom) I put mine just inside the rear door as shown. I then simply slam the rear door on the cable, which runs out through the bottom door seal. I figured that the small break in the rubber seal won't let rain in as it can't travel upwards (and I was right!) and found that there is plenty of space for the cable when the door is shut. Temporarily having the rubber door seal squashed by the cable doesn't seem to permanently distort the seal either, but to be honest a tiny gap in the seal at the bottom wouldn't make any difference anyway. Being careful, I covered the section of my mains hookup cable where it goes through the door seal with a layer of plastic automotive conduit, for extra insulation and wear resistance. No probs at all after 18 months and lots of trips away.
Could be worth you trying with a bit of cable to see if that works on a Custom as well?
In my old van I put the socket under the floor and drilled a hole through the floor pan for the cable. This was a pain in the @rse, as I would have to lie on my back on the ground to hook up. One other option people sometimes opt for is under the bonnet, but then its a relatively tricky cable run through to the loadspace.
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