Dan Fromm said: "You may think you don't need more gear, but to get full flexibility you'll want a manual flash with power level control. The Vivitar 283 with VP-1 is ideal for this. Out of production, very available used."
It's amazing how Dan's genius closely parallels my own.
I bought a used Vivitar 283, and a VP-1 attachment that allows it to be used off camera. There are plenty of filter hoods that fit into the flash end of the 283, and they will allow you to stack a diffuser filter and a neutral density filter. With an adjustable flash bracket to hold the 283 off to the side, you cn use a reflecting card on the other side for fill light. This rig works equally well with my old Pentax KX and with my newer Pentad *istDS. I will probably try it next on my Retina IIA (since that 1950's vintage camera offers X-synch.
Yes, you already have a flash, but the Vivitar 383 gear is ridiulously inexpensive on the used auction market.
Here's a gear bag zipper from a hand-held DSLR, exposure completely controlled by the Vivitar 283.