Motion: how to add a mask to a filter (but not mask footage below it once back in FCPX)?

In Motion:

How do I apply a mask (circular mask) to a filter (gaussian blur) ?

I've tried everything but when the Effect is brought into FCPX the mask, masks/blacks the footage below it. I just want the mask to mask the blur as set in Motion.

Basically the same as adding gaussian blur to footage then adding a mask to the gaussian blur - that works, but how to mimic that in Motion?

I've hit on proposed solutions, but in Motion they all entail duplicating a footage layer then applying a mask to that. I don't want any specific footage in Motion, I want an effect I can apply to any footage once back in FCPX... gaussian blur with a mask.

I want to blur toward the sides of footage, this can be done in FCPX, yes, but I want an effect that contains the slider controls I can add in Motion. This seems like it must be simple, add a mask to blur in Motion, but the mask not mask footage below it once in FCPX. Any ideas?