I've used the rollei hoods on my mat. Yashica hoods an be found but are harder to source than rollei hoods. I pretty much use a hood for outdoors shooting regardless of light/weather conditions. They protect against both unwanted flare as well as (partially) against any inclement weather.
You'll want the Bay 1 hood and they are available anywhere from 17 USD to 100 USD on popular auction sites if you can't source one from locally or places like KEH.
As for your shots - it's really hard to tell from a browser, but it looks more like negative density issues than anything else. Try metering a full stop below the films rating (i.e. shoot 100 @ 80 or 50, 160 @ 100, 400 @ 320 [some colleagues of mine will meter 400 @ 200], etc.) to ensure you get the best negative density. From there you can make a better determination.
Also bear in mind your shooting with a 50 year old lens - NC films produce better results in the mat than VC in my opinion. Todays color film is geared for double coated lenses and it's hard to balance the VC films.
