AE5 how to restrain stroke expression within a shape?

I'm working on a banner anim for a TV shop website. Size is 570 x 82. It's a 10-second anim I plan to reel/repeat.

I have 4 text layers, all the same text, with some basic color, glows, and opacity shifts. All good here.

On text layer 4 I added Generate Radio Waves with a modulating RGB color stroke. This stroke speeds up then cuts off at 5 seconds, for a nice "electron blizzard" type effect. However the stroke fills up the entire 570 x 82 space, and there seems no way to shrink nor constrain its size. Scaling the layer did nothing... I added Mask and tried selecting Mask for the Radio Wave Type, but it doesn't seem to make any difference, I can't seem to scale the stroke down.

I tried creating an elliptical shape layer, but I couldn't get much to happen with it.

I imported a color ellipse psd as layer 5, sent it to the Back, and tried to make it a mask or matte; I thought I could restrain the layer 4 Radio Wave stroke expression, to within the ellipse psd of layer 5. I can get close by using this psd layer as a Track Matte, but I lose the cool RGB of the Layer 4 stroke; the track matte has only 4 choices and all mutilate or invert or just disappear the stroke color from layer 4.

I'm a relative newbie, I'm sure there's some way to get the stroke effect constrained--- I even tried adding a Drop Shadow to the back .psd layer: still no go. So in brief, the Goal: create an elliptical banner animation, w/ several layers, one of which has a Radio Wave stroke effect animated--- and which effect, doesn't go beyond the elliptical boundary of .psd layer 5. Or, find some other way to constrain that crazy stroke. Hoping some wizard out there can help---
Charlton
Houston