[Aussie Mike]

I can see the problem. The feed in pipe is pointing straight at the drivers side. How about putting a partition in the plenum just after the feed in pipe that fills the bottom 1/2 of the channel that would force some of the air to do a u turn up the other side of the channel and even out the balance. Try cutting a piece of ice cream container and taping it in place to work out the right height.

Cheers