Questions about Elbro Cable bridge

This cable bridge is kind of huge. Not so attractive in a flat (e.g. to lay cables more inconspicuously and orderly on the parquet floor). Especially in the width (70mm). I imagined something more filigree. Product fits well for 2 power cables and to lay thinner cables in the triangular side channels. For only 1 power cable it is def. too much. Returned unopened.

I received the package today. But I am now surprised that there is no opening on the inside of the rail. How can I get the extension cable into the rail?