Depending on how critical your framing needs are, you could get by with something as simple as a wireframe viewfinder. Essentially, it's just a small hole to look through and a sensor- (or film-) shaped rectangle on an adjustable boom. There's usually some sort of composition guide (either a rule-of-thirds grid or a simple crosshairs indicating the center) strung across the rectangle to aid in framing. You simply adjust the length of the boom extension so that the field of view through the wireframe is the same as it is for the lens.

You may be able to adapt an existing viewfinder (they were made for Speed Graphics and similar "press cameras", as well as for pre-viewfinder Leicas*, etc.) -- the used market is full of them -- but it's not too terribly difficult for people of moderate handiness to make one for themselves. If you are zooming during shooting, then a glass or plexi panel with a series of concentric rectangles instead of a wireframe would give you a good-enough framng approximation, provided that you have a decent feel for the zoom.

*The first couple of generations of Leicas had no proper viewfinder. The original was a "point-and-hope"; the second iteration had a rangefinder but no real framing viewfinder.