Before I'd spend money on a tackle specific pack, I'd take a similar sized day pack you have or can borrow, and rig it so you can attach it to your portage pack.
I think you'll find, or at least I did that a pack big enough for your gear obscures your view of your feet, which in rough terrain means you can't see where your stepping.

I use a old day pack for a first-aid/rain gear pack. It's about the size of those tackle packs you see for sale.
I found out that if I hung it across my chest, it was an accident waiting to happen.
I modified my carry rig. I bought a Big Carabineer and hook that thru the packs top carry handle.
I then clip that to the D-Ring on my left shoulder strap of portage pack. I use my left arm to pull pack into my body as I walk.
I can see trail, and IF I need to drop pack for whatever reason, it's easy to unclip.

Not for everyone, but I'd do a dry-run with what you have and see if that type of set-up is really for you?