I use a Sightron S11 6x42 fixed scope on both my .243W deer rifle and Air Rifle (Air Arms S400 Carbine) and both do the job just fine, hold their zero, and are multi-coated, waterproof and gas-filled.
The 'fixed' S11 hunting scopes are not produced anymore (Just a 3-9x42 in the S11 series now) it was one of their more basic lines, but if I could get hold of another one or two, or the fixed 4x32, I would buy them immediately based on my experience with them.
The current
S1 hunting series also looks good - it ticks all the required boxes: Multi-coated, gas-filled, waterproof, affordable.
Buy direct from the USA online (Pay by Credit Card) and you'll get a better deal than here in the UK, and it is likely to arrive quicker as well.