Hey guys, another question

I am trying to modiefy a railing into make a fence about 2500..
I am trynig to have a vertical L 50x50mm every 3m and i dont want a continious top rail... When i set this top rail to No i get error about cant create railings...
How can i solve this...?
I am actually trying to do this at the end.. Sorry for the paint drawing
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