I'm not exactly sure what you're attempting to do. Just about anything you can do in AE you can do in Motion... sometimes you have to think about it and execute it in a different way... usually it's very much easier to do in Motion -- so much less complication than AE.
If I start with something like this:

I created this by using the Checkerboard generator (fills entire canvas/background), loading in the Library > Shapes > 5-sided star. I duplicated the 5-sided star (for the red outline) and used a filled version for the shape-mask applied to the Checkerboard (drag and drop the filled star onto the checkerboard to apply it as a mask.) So, for the layer order, at the bottom I have the Checkerboard with the shape mask and the 5-sided star with the red outline only on top.
All of these items exist in a Group. I selected the Group and selected the Group inspector. I set Fixed Resolution and adjusted the width and height to just fit the points of the shape.

Bounding box looks like this in the Canvas:

When you select the Distort tool, you will notice it has absolutely NO effect on this... so... with the Group selected, type 'K' to create a Clone Layer. Now, the bounding box of the 2D Fixed Resolution group will be used for the Distort (a.k.a.: Four Corner) tool and you'll get the appropriate appearance distortions around the shape area and not the entire plane of the actual source materials:

There are also several Distortion filters that can be applied to this clone for more sophisticated distortions (which are applied to the object *before* the the Four Corner distortion).
To help with "softness" issues, start with a larger object and scale down (the clone layer object)... you'll get a better results if you have to do a lot of distortion.
While I'm thinking about it -- any animation you apply to any of the objects within the original group will appear (in real time) in the Clone - it is not a fixed object. Doesn't really apply here, but Clones are a very powerful feature in Motion that gets overlooked quite a lot. They are at once, an object, a rasterized movie of its source material, a placeholder for individual frames (think emitters and replicators), etc.
HTH.