I have no experience with either of the two cases you mentioned.
I use a Granite Gear map case that isn't waterproof by design as it just has a fold over top secured with Velcro.

I've had great luck with using zip-lock type plastic bags to put my folded maps in. I found some on line that are 2 gallon I believe. Laid flat, they fit the clear portion of my map case. I fold and stack my maps in relationship to my line of travel. I've found that by squeezing out all the extra air and zipping them, I end up with a waterproof map case.
You could do the same with your old Chinook. To avoid the flip and fold, you could put one map per zip lock bag. I think you could do it so you just flip the bag to continue on with the line of travel.
I'm using McKenzie maps but unless your using just one map like a Chrismar, it should work okay.