Square solar filter?

[zhgutas]

Hiya all.

Short version of the question - can I make my solar filter square, and will that affect the view in any negative way?

To elaborate: I have bought a piece of Baader astro solar film (10x15CM) for my ST80, but then decided, why not use it for my ST120 instead.? The problem:  there is, of course, not enough material to make it a full aperture circular filter, ant I would prefer not wasting any by making a mask with a smaller circular filter.

Is it possible to mount a full square piece, that way, gaining several square centimeters of aperture?

I'm attaching a really crude drawing of what I mean.

filter.bmp

Thanks for help!