Start with an Effect project.
To the Effect Source **Group** : add two Filters > Distortion > Bulge filters. You'll get Bulge and Bulge Copy, which is fine for this.
In Bulge Copy, right-click Amount and Add Parameter Behavior > Link. Right-click Scale and add another Link. Right-click Center X and add a Link, then right click Center Y and add a Link. For each of these links, drag the Group into Source Object, then select Filters > Bulge > and the corresponding parameter with Compatible Parameters. Go back to the Center X link and change the Link > Scale to -1.0. This will "reflect" the effect of the filter across the vertical center of a project — one Bulge will affect the right side and the reflected one, the left side - simultaneously. (There's more - below.)
Open the first Bulge filter and publish: Center, Amount and Scale.
Select the Effect Source and publish Properties > Transform > Scale and change the name in the Published Parameters list to something like "Scale Source" to differentiate it from the Bulge Scale.
Before you save the Effect, select "Bulge" and Check the Publish OSC parameter (but not the other one - you only have control with one OSC in FCPX since the two are linked together.) You should also set the Center of Bulge to the center right edge of your project (if it's a 1080 project, then X = 960, Y = 0 -- you can dial down the disclosure triangle and set X to 1.0 and Y = 0.5, instead.) At this point, if you check out the location of 'Bulge copy', you'll see it is way to the left. The reason is that the "true" default location for filters is 0, 0 and the center of any project is 0.5, 0.5. When you set the original Bulge to align with the right edge (1.0 or 960 for 1/2 a 1920x1080 project width) the negative value created with the link places Bulge copy at about -1920 or way off the canvas. So the Link for Center X > X offset (at the bottom) needs to be adjusted to -1.0.
Also before you save, I recommend the other "default" published parameter values be: Amount = 1000, Scale = 1.0 (or not initial effect) and Scale Source = 100%.
This is the exact formula I used for my Fisheye Redux effect that I used above. However, I also used my Lens Distortion Fix effect to straighten out the verticals. Although I believe you can do the same with the Transform > Distort function in FCPX, my effect works just a little bit differently, so YMMV.
If you have clips with the same distortion over and over that require the same basic settings all the time, once you set up this effect in FCPX for one clip, save your settings as an Effects Preset with a meaningful name. From then on, all you need to do is apply the Effects Preset and the settings will automatically apply. FCPX will quite happily load the Fisheye reducer effect and apply those preset parameters all in one throw.
If you run into trouble, let me know. Even though this is a "simple" effect (not a lot of parts) I really wouldn't rate it as "easy".
