This is what I've done with western trailing types. No trellis needed, just a few stakes. Triple Crown stands up better so should work.

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For my Triple Crown (and for my boysenberries) I built rectangular frameworks (think box kites) out of 1/2" PVC electrical conduit (the grey stuff) that are about 6 ft. tall. The size of the square base of the framework is based on the diameter of the pot at its widest. The pot sits on the bottom of the framework or cage. This helps stabilize the assembly by adding weight (unless you forget to water!)

So far I am using the verticals to tie up the fruiting canes while guiding the new canes up the open center. When the fruiting canes are all done, I will cut them out and tie the new canes. That means I am maintaining 4 fruiting canes per plant (5 gallon containers).

Seems to be working OK so far. If I pot up (which I probably will) I just need to change the length of the horizontal bars (I did not glue anything).

Wow do those taste good containerized?

Persian:

Yes they were good this year. Especially Black Diamond and Black Pearl, both thornless with big sweet fruit. I also like the fruit on thorny Obsidian and Metolious. But would choose the thornless for certain.

Triple crowns tend to get the white druplet problem caused by sun damage to the ripening fruits. Your container plant has the advantage of being mobile, move it to a shadier spot if you ever experience the sun damage.

Also, your plant was probably not a "seedling", but rather a "start".

Thank you, everyone, for your help!

Fruitnut, especially you! That looks like a brilliant idea!

And yes...start.

That is really cool, I've never seen a blackberry containerized like that.