It looks like the canes are supported fine right now, although it is not apparent how. It looks like the planting is centered on a window.

If I were doing this, I would drive some metal t-posts each end of the wood-lined bed, perhaps one in the middle. Attach top of posts to 2x2 horizontal wood by drilling holes and using screws. drill more hole in the posts and run a wire (or more wood) horizontally about halfway up. The trellis only needs to be 4 or 5 feet tall.

In future years, cut the tops of the new canes when above the trellis and that will cause horizontal branches to form, easier to deal with than with very tall canes.

The mass is held up with three metal clips at the moment so for the most part, it is self supporting. The problem is that it is too deep. I would like to splay it wide along the 25' wall.

By keeping the main canes at a shorter length (often new canes are 5 feet tall by sometime in July), the laterals may well span the entire 25 feet, that would only require 10 feet in both directions from the current clump.

If your canes remain spindly like that, you can just let them grow without cutting to a specific length, and arch them sideways down the trellis. When bundling canes, limit bundles to 3 or 4; bundling more may hamper growth of fruiting spurs.

If the main canes are too thick to easily arch and keep below 4-5 feet high, then tip-pruning and lateral formation is best.

It looks as if there is more to this than I thought. I read that one does not trim in the first year. This is the end of the second year. I read that pruning is done in the spring and again shortly after harvest when one can trim out those that produced fruit as they will not produce any more. This produces the vines for mext year's harvest.

I believe that I can brace these canes left and right along the garage and then maintain the growth off of those for fruit. Therefore, I figure these base canes can be 5' above the soil and the new growth will grow down from there. Does that make sense?

If tis is a good plan of action, then when is a good time to do this arrangement and pruning and back to the original question, what do I use to hold the vines? I can purchase the U-shaped clips used to hold PVC piping and hold the horizontal canes, yes? The clips are held in place with two screws and allow for a 1" diameter pass through. Should this work?

The leaves will drop soon so looking at the structure will be easier and best of all, it is all thornless!

More than you thought; keep in mind that blackberries, in favorable conditions, are a very weedy and vigorous plant and can tolerate various pruning methods while still producing a crop, so there is no one correct way for the home gardener.

The plant will put out new canes whether or not the old canes are pruned.

The new lateral growth of tip-pruned canes will be at random angles but does not grow "down" from the cane tip, but sideways from multiple points.

Prune out old canes after harvest and tip prune new canes when maximum desired height is reached.

Are you going to attach the clips directly to the garage wall? That would require over 100 clips that would need to be removed and re-placed every year. I never recommend training blackberries directly against a wall and ripe fruit will stain it. Go with the t-posts in the ground a couple of feet from the wall and attach canes to them and to each other.

It sparks a couple of questions.

The plant will put out new canes whether or not the old canes are pruned.

It looks as if I will be taking the canes that are already there from this year and bracing at about 6' high horizontally along the garage. I figure that I can continue to use these canes year after year, not unlike grape vines. It looks as if sooner or pater, I may have to clip these out and use new ones as teh central feed.

It will be interesting to see how the new canes are produced. Since they are flimsy at best, I figure they will drop down.

The new lateral growth of tip-pruned canes will be at random angles
but does not grow "down" from the cane tip, but sideways from multiple
points.

In this case, sideways is up or down, no?

Prune out old canes after harvest and tip prune new canes when maximum desired height is reached.

In this case, maximum width.

Are you going to attach the clips directly to the garage wall? That
would require over 100 clips that would need to be removed and re-placed
every year.

I can imagine using no more than 6 clips as the one in the picture is held up, although sloppy, using only three.

I never recommend training blackberries directly against a
wall and ripe fruit will stain it. Go with the t-posts in the ground a
couple of feet from the wall and attach canes to them and to each other.

I can see your point but I can only imagine that the canes will grow against the garage anyway without considerable maintenance anyway. I am not all that attentive of a gardener in the first place so it is going to happen any way.

Still, no one has indicated any method to actually hold the horizontal canes in place for support. I can use steel poles with some kind of wire to wood beam across but am still at a loss for actual attachments.