The soft box idea would probably work OK, though the strip box may or may not, as it is so skinny. Remember, the lower half will have the light and stand blocking part of the light coming out. The bigger the surface area, the less of an issue.
Generally, my only concerns would be keeping the Black Outer Cover taught against the Diffusion Fabric once a hole is cut into it. Also, that is a lot more volume to deal with (especially with the strip box) than a dedicated softbox. And you will not be able to use a grid.
Lastly, the 64" measurement is across the arc of the PLM. The open face is about 54".
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