Re: 338 RUM is a tack driver and a forehead killer!
This is just something I've learned to live with since my start with a light 300 Win Mag while I was but a 130 lb kid nearly 20 years ago.
My secret is to mount the scope really far forward so my neck is "pre-stretched" when reaching up to it. After a little bit, you get used to it. That's what has felt natural to me for years now.
I just weighed my 300 RUM in its current guise at 9 lbs 5 oz and I've shot 290 grainers from it at 2800+. Its current scope only has about 2.75" eye relief to the actual eyepiece that would bash you (the ocular lens is set in a ways). But the scope is mounted far enough forward it pretty much can't get me. Longer eye relief helps but I've found it doesn't help enough if you still mount the scope too far back--even if you back up your head, recoil will take up all that slack.
In short, move the scope farther forward and you may fix your problem for free without adding something you don't want.