That inexpensive bamboo fencing stuff which I've seen at Home Depot could shade them from the side. Or trellises, planted with some fast growing vines. Overhead you might need to build something and use that green shade cloth or some lattice overhead. You could make them a little lath house, sort of like a giant milk crate.

Why don't you observe the sun this summer to see exactly how the it hits them and what you think you'll need next summer.

Sounds like a lot of work, but if it's going to be years before they are shaded, it would be worth it.

Socks, thanks for replying. The 2 Rhodedendron Maximum are not really growing fast so far and may only grow a foot this year, much less is expected though. One of them will soon be okay without the crate, my Chestnut oak is growing fast, and will soon shade the Rhodie a good bit. The other oak is slower so far this season. The area that currently has no rhodie I should wait until the oaks get much bigger or plant another closer to the spot the Rhodie will be in. I could plant a commercial rhodie that can handle more sun too. But the wild Rhodies are really graceful and beautiful.

Socks, I am happy to say that now (3 years later) I am able to have 3 Rhododendron Maximum planted under 3 oaks, that are large enough to shade the 3 Rhod maximum's. But, I am thinking using Bamboo fencing to make a new area in the garden sounds nice. I currently have a clumping bamboo in my yard. These bamboo culms have some new growth that is about 1 1/2 inches per culm. But, this Bamboo is far from invasive, so I hope nobody gets excited and flips out over this bamboo.

I have 3 Rhododendron Maximum as a already mentioned, under 3 different oak trees as mentioned, but, I want to mention that Rhododendrons need a shady area, with good rich soil with lots of organic matter in it. If you buy a potted Rhody get a bag of compost to transplant it into your home landscape.