It would certainly make it easier and a better/tighter connection. - Nylon Tie Tensioning Tool
Or use a couple channel locks or other tools to pull onto the tail end piece, and another to push against the head of the tie where it’s clicks. This likely won’t get you the best seal and secure connection but ya also understand not getting a tool for a one time use.
Is a zip-tie gun excessive/unnecessary for cable management? I'm thinking about picking up this Klein 86570, but unsure if I really need one. I'll also be working within a SFF case (specifically the NR200), so space and maneuverability may also be issues.
I've seen several places where they use one of these tools to tension zip ties, and they can pull them pretty tight. Definitely not recommended for cables
Something like this one by Klein tools.
I have a gun style one I use too. It's actually more convenient.