Getting the flash off camera is a MUST. You want to be able to fill shadows from the sun. If you just shoot straight on, you simply fill both sun lit areas as well as the shadow. It does almost nothing in the end. Perhaps slightly better than nothing, but not as useful as moving the flash to the proper angle.
A relatively larger light is always helpful for diffusing light. Sadly it comes at a cost of distributing output across that larger area and thus decreasing its apparent power on the subject. So you may want/need a soft box with more than one flash head in it.
The bigger the better, but how much can you carry to the location and how much money do you have?
At some point you're going to have to go with battery powered studio strobes if you want a lot of diffused flash in bright sunlight and a band of assistants to carry it all.
What type of event?
You're likely going to want an assistant if subjects are moving at the event. Otherwise you're moving a weighted down light stand (even a mild wind catches umbrellas and softboxes incredibly well). A soft box of most any useful size is going to be difficult to handle hand-held by yourself.
I have a 24"x24" soft box for my flash. It is a good compromise on size. It is still small but works well within a few feet with one or even two flash heads taped together and remains more mobile. One can use it hand-held if necessary.
These small softboxes for off-camera flash can run hundreds of dollars from top name brands. But you can also find them on Amazon for under $40.
I picked up this one and it seems decent. At least it is a good start to see what works without breaking the bank.
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As a side note: With Pentax, unlike other brands, you are limited to the shutter sync speed of 1/180s. So if you are trying to balance the sky, you will likely need to make sure to use High Speed Sync. This cuts the effective power of your flash. So a second flash head is a good idea. There is no hyper-sync
The new cactus V6-II trigger is great for HSS implementation. The older V6 never worked well for me.
---------- Post added 10-24-16 at 11:43 AM ----------
FYI, something like this even works just as well but much cheaper than the Gary Fong stuff.
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If you're even only a little bit crafty, you can make your own with some white foam sheets from the craft store for under $2. I have one almost permanently attached to my flash. Its about 16 times the size of the flash head itself so it is softer at close range. (doesn't do much at a distance -- and neither will Gary Fong or any other tiny little diffuser)
This one works better indoors so some light bounces off the ceiling, as a good portion of light goes straight up.