Windowing an image: the easiest way?

i've scanned through entire premiere pdf , there nothing obvious , simple how "window" image. i've visited several websites; again nothing obvious , simple. so, i'll have ask experts here.

 

what want create rectangular area – window – within 1920 x 1080 frame, within clips in sequence appear. part of clip outside window, black.

 

i don't need instructions how it, know name of function in premiere allow me create "window", can up.

 

i have tried me obvious, inverted, method. mask:

 

1. create new item black video (p255 of pdf).

2. test purposes, set size of black video to, say, 800 x 1080, mask full height 800 wide.

3. drag black video layer above clips.

 

my theory parts of clip lay under black video not appear. , worked – sort of. problem was, there unexplainable (to me) offset. boundaries of black video (as shown when clicked on effect controls > motion) different how clip underneath being "windowed". offset not constant, vertically or horizontally, , seemed acting drop shadow in indesign, light source set @ angle.

 

any suggestions how "window" clips?

 

 

aside: within indesign, these "windows" thinking of created automatically every time set new text frame. done thousands of times on years. dump image in frame, , frame forms window image. no part of image inside can appear outside window.

have tried track matte key effect?

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