It is not going to be useful over the pack so I need something to cover the pack. Not sure what is good for that.
You could use a pack cover. Just size it right.
This is the Integral Designs poncho I use. Got it cause it snaps under and covers your pack. A little pricey but I've had mine for a few years and so far so good.
I think I paid $3 for the poncho. Paying $100 for a pack cover seems a bit excessive. If it comes down to it. I may just throw the poncho over me and the pack, and accept that the back of the poncho will ride a bit higher. Or maybe I will make a pack cover out of something.
The few times I have walked in the rain in a poncho I found that while they didn't breath, I could flap my arms around every now and then to bring in fresh air and that was adequate.
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Warning - I am not an expert on anything having to do with this forum, but that won't stop me from saying what I think.

Bob