Are they likey to hold moisture in and promote rusting ?

It has a stock flap to protect the stock.

It gets humid here in the upper Midwest.

You shouldn’t store a firearm in a leather holster or scabbard long term, particularly in a humid area.

Yep. Storing a firearm inside anything made of fabric or leather for extended periods will cause rust.

That goes for holsters, "gun rugs", scabbards, etc.

What are you wanting to use it for bendable?

As has been said, long term storage inside anything is worse than open storage in a humidity controlled environment, even worse than a non controlled environment all things being equal.

If you want one for temporary transport an an ATV or similar then let us know.

A friend wants one for his lever action for storage and transport too and from the range and hunting venue.

Thank you for taking the time, will let him know.