Until I got to your last sentence I figured you had warped, stamped steel covers. Now, I would suggest that you may have poor quality, cast aluminum covers. And assuming you're getting your leakage around the bottom of the cover.

You are correct about proper torque, i.e. not very much. I like the viton gasket material (most people call it rubber though it's not) with metal core. If you need to keep the covers you have then you may need to put a bead of RTV on the aluminum side since that's where the distortion is. Also, next time they need replacement, you can scrape the sealant off the car instead of having all the little pieces going into the engine.