I've owned five rigids (I still own two) and have never dinked around with minimalist rack systems. Suction cups, scrap aluminum, ladders - all of these will eventually cause glider or vehicle damage when hauling most gliders - and especially rigids. Here's what I've done for the three vehicles I've used over the years for hauling rigids:
First, one needs to look at the structural support the vehicle is providing. I've always used square steel tubing. It's easy to use in fabricating the rack, and provides the best horizontal supports for adding the padding system. If you're using an existing structural system like a lumber rack, you may have to work with round-stock metal, but at least you've got the structural strength in place.
When hauling rigids, it's especially important to not rely too much on the vehicle's roof for support. I only use one intermediate support point to be mounted on the roof. Don't use a lightweight aluminum Yakima or similar - they just don't offer the support desired for a rigid. If you don't have the capacity to come up with a steel roof rack, a wood stiffener mounted to the top of the rack might work - assuming the clamping system is sturdy enough. I've always used front and rear racks to supply the majority of the support, which can be reliably mounted to the frame in some manner.
Which leads me to the padding system. I've used this design for the past 15 years and never had the slightest problem with rigid wear or damage. All the other pilots who load on my vehicle (rigid or flex) claim it's the best they've ever seen.
I start with a redwood 2x4 - redwood is a relatively soft wood, and known for its resistance to rot. I drill two 1/4" holes through the board to install the mounting bolts before I add any padding. I use 1/4" galvanized bolts and pound them into the holes so the heads are recessed into the wood, so they won't easily turn when threading on the nuts when installing.
I next cut my first layer of padding. It's a dense foam, just over 1/2" thick that can be found at craft stores and (I think) Home Depot. After cutting it to the shape of my 2x4, I use a spray adhesive to mount it. I next use a less-dense foam, about 1" in thickness I got from a fabric store, and attach it with spray adhesive as well. While at the fabric store, I buy a good-quality vinyl to use in covering the assembly.
I wrap the vinyl around the wood/foam assembly like one would wrap a gift. I cut some of the excess vinyl to decrease bulk in the folds, and use a staple gun to hold it all together. I cut two slits for the bolts to protrude through, and I'm ready to drill the mounting holes in the steel horizontal support members. I thread on a galvanized nut with a lock-washer, and the pad is complete.
This type of system offers the best support for rigids - a sturdy structural which won't damage the vehicle, and pads which will never bottom-out. The only problem with this system is the critical look you will give your buddies' racks when loading your precious wing onto their lesser racks.