Here's a suction-cup-based rack for my Volkswagen Polo. I hope this helps you design a rack for your car, as the other examples helped me. I live in the UK and you might want to know where I got the padding materials (in a word, Homebase).
The front rack is an aluminium frame attached to the bonnet via suction cups and secured in place with rope. The rack was cheap to make and is fairly sturdy, though don't think the suction cups will hold well. It should be able to take 3 gliders. For the frame, I bought a 5m length of aluminium L-section which was 25mm wide and 3mm thick for about £20 from a local shop. You can order this easily and cheaply enough online by the way - I nearly did. Box section of the same cross-sectional area would have had better bending stiffness but I thought the L-section was easier to fit together. What I hadn't considered was torsional stiffness and I had to add the second horizontal bar to stop it twisting. Anyway, if you have a hacksaw and a metal drill, then it's easy to bolt something together (pack of hex bolts from Homebase for about £3). The way the frame is attached to the suction cups is a bit of a bodge but L-section made it easy enough.
The padding on the front rack is the same as on the roof bars at the back. I've used flooring underlay from Homebase and I highly recommend it. It's white/clear and came on a roll 15m long and 1m wide, is a few millimetres thick and cost £15. The alternatives seemed to be pipe insulation or pool noodles. Pool noodles seemed to be the material of choice and I would have used them if I could have found any in the UK. I think the problem with pipe insulation is that it's hard to find any very thick and that it squashes down too much so that it "bottoms out" and therefore doesn't get rid of the pressure point. You get a reasonable stiffness from the flooring underlay without going too thick. And because you wrap it, you get to choose the thickness. I covered it with stuff called Gorilla Tape, also from Homebase, about £10 for a large roll (which I completely used up). It's really tough and claims to be UV and water resistant. It was recommended by someone else to me and seems pretty good.
These are Silverline suction cups bought via eBay for about £15 including postage. I think they're designed for picking up sheet glass and you need a relatively flat section of bonnet otherwise you won't get them to grip. There's basically only one place I can put them. I have drilled a small hole in the top of the handle. A small bolt sticks out from the frame and goes in the hole. The frame is free to rotate in all directions in the suction cup. There is a cable tie to keep the suction cup attached to the frame when not in use. When in use, the rope tension and the weight of the frame and glider keep the frame and suction cup together. I was going to try and have some kind of shackle/clevis arrangement but couldn't find anything suitable before deciding this was good enough. All the weight of the glider gets transmitted via a sharp nut to the plastic suction cup handle, which isn't great. I plan to get some kind of rubber bush in there at some point.
Ropes tie the frame to both the roof rack and the front tow loop. The frame must be free to pivot relative to the suction cup because otherwise it's like a big lever and would unsuck too easily. You could have the glider itself hold the rack in place, but from my experience with another rack, I recommend against it. You have to strap the glider down pretty tight to make sure there's enough friction to stop the front rack going anywhere which would only cause excessive wear. The advantage of ropes is that they compress the suction cup to the bonnet. Because the suction cups are not reliable, you should probably think of them as a big rubber pad. You therefore need the ropes to hold it down. I don't rig the ropes like in the picture any more. There are four sections of rope, each tied separately. Imagine that the suction cups will fail and rig whatever rope is necessary to hold it in place.
The supports aren't as far apart as Will's Wing suggest, nor are the supports as wide as they suggest, but I'm pretty happy that I won't wear the sail out from transporting it like this. Plus I can get enough gliders on my small car to make it a retrieve-mobile. It cost about £75 in all plus £75 for the roof bars. Pretty cheap but it was more effort than I anticipated. I hope this helps someone make a rack themself.
- If you are in the UK, use flooring underlay and Gorilla Tape from Homebase for the padding.
- You can't rely on the suction cups holding.
- Making the frame was a pain. I've seen another made from a single piece of aluminium tubing and bent with a pipe bender. I didn't take a close look, but I bet this was sturdy and much easier to make.