I don't know which clematis you're growing, hopefully its one of the hybrids and not a species such as montana, but, so long as you don't mind screwing two battens at either side of the arch vertically, just go and buy some white clematis mesh, and attach that from one side to the other on the battens you've put up. Clematis is not a twiner nor self clinging - it climbs by means of thin, modified stems on leaves, which need something no thicker than a bamboo cane to twine around, so it wouldn't have climbed up a trellis anyway. Clematis mesh is available in white, brown or green, can be bought in rolls in places like Wilkinsons, or in lengths from the garden centre, though they usually only stock green or brown - I'm only saying white so it blends with the wall, which colour you use is up to you.

By the way, the photo IS the right way up...

Thanks for putting me right B, as usual I had no idea. That sounds a great idea. And simple. I think I want a clematis that has small flowers and is red in colour

Look at Clematis texensis ones then - they have bell shaped flowers rather than large, open flat ones, and there are some reds or deep pinks to choose from, in particular, Clematis t. Gravetye Beauty and Ladybird Johnson, link below to pics on Taylors website. Prune group 3, which means you cut down to about 8-10 inches from the ground in midwinter - which is good, because then you're not looking at dead growth all over the arch all winter...

http://www.taylorsclematis.co.uk/Clematis-texensis/

One other thing - its probably wise to put a batten at the top too, and fasten the mesh to that as well, otherwise, with the weight of growth from the clematis, it might start sagging at the top.

As for having 'no idea' Hank, in my experience, everywhere I've worked, the owner has erected trellis for a clematis and wonders why they keep having to tie it on...

Thanks again, B, I'll study that very carefully.

I'd use screw eyes with fishing wire - much easier to de-tangle when you need to prune the clematis and you can extend it according to the rate at which your clem grows and the direction that you want it to go.
Clematis Westerplatte is a striking red and should flower for the whole summer into autumn but the flowers are quite large.

Well, Urbanite, you'll note I've mentioned a Prune Group 3 clematis - I've done that for a good reason. Being able to remove the top growth almost to the ground once a year not only keeps it tidy and within bounds, but also allows for any maintenance work such as repainting and the like on the wall and the supports, because the mesh is easily removed one side for such a purpose - its usually attached with staples. There are plenty of other prune group 3 ones available, though a proper red is hard to come by, specially in smaller flowers. C. Westerplatte though, is a prune group 2, which means there'll always be growth over the wall... But in the end, its up to Hank whether he uses what I've said or what you've said in terms of supports... and clematis, come to that.

Thanks guys, more food for thought.