Don't come on here very often anymore, but it seems like this topic keeps resurfacing. My solution was this, IMO there is no better way around it. Sure it requires cutting a hole in the floor, but that is no problem whatsoever, even if you would sell the van, just cover it. No water from the rain ever comes in either. Little bit more road noise, but not too bad. Just need to mount the winch guide properly.
http://www.cleanitup.co.uk/smf/index.php?topic=76893.0Tweaked here and there but been using this for ages now, wouldn't be anywhere without it. For the last ~2,5 years I've never had the reel out of the van.
Got Gardiners new lightweight reel with 6mm microbore and I've got major backlash if I don't gradually slow it down, it rolls off that easily. Reels in effortlessly. Safes so much energy and time. Get the pole out, start walking.
Needs new rollers on the cabinet mount, needs ideally metal ones made up. The surface of the winch guide rollers looks a little worse for wear, but the plastic bearings are holding up just fine. Needed some ballbearing grease to stop some vibrating noise. Also upgraded the reel clamps for lockable clips that screwfix sells. Other then that it's working brilliantly still.