Rattling Lever

Looking for some advice for my rattling lever!

Fitted a set of shimano r400 levers, one for the front brake, the other just for symmetry/comfort.

The unused lever rattles like hell I'm guessing as i've not run a brake cable through it. Tried taping the lever up to fix but I think it's inside the hoods and not the lever itself rattling. Any ideas on how to stop it short of filling it full of glue :D

I was thinking of running a cable through it and tensioning it somehow to see if that helped but not sure how i'd tension it up.

Put an off cut of brake cable inner+outer into it, pull tight and tape under the bars as you usually would at the end where you want it sort some way of pinching the cable so it doesn't move.

Get a tt brake cable joiner and use either lever to control the one brake.

No rattles and double the braking options.

This is a great idea didn't know such a thing existed. Can you link me to something as not sure I can find it'll the correct thing.

How would you pinch it though to keep it taught? I don't reckon taping it down will do.

Taping it down would be fine. Until you squeeze the lever accidentally and pull it through.
Bluetac? Otherwise cabling it to your front brake like DJ says would be ideal

https://www.sjscycles.co.uk/cables/jtek-doublecontrol-l/

One of these? You essentially make it like a double ended cable the exact length of the outer cable.

https://www.sjscycles.co.uk/cables/solderless-barrel-nipple-55-mm-x-7-mm/
https://www.sjscycles.co.uk/cables/d2o-solderless-pear-nipple-7-mm-x-11-mm/