If this is only a once a year thing I would probably just shove in some coil spring helpers. Nivomats are too expensive for a one time thing. On the other hand if you are loading and unloading stuff a lot then Nivomats might be the way to go. I have Nivomats on my 855R and so far i have had no problems with them at all. They have worked as promised with whatever I put in the car.

The coil spring helpers I used in my last wagon look kind of like giant square screws. They sit between the coils and hold them open. Alternatively there are some other fancy adjustable ones with nuts and bolts that work too. Just make sure that you put 2 helpers in each spring and that the 2 helpers are in the same coil but mounted 180 degrees from each other. Don't mount them one above the other.
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Nathan Valles, Black 855R, Tint