The "L" bracket seemed to allow the bearing to tilt slightly. I noticed black shavings coming off of the belt where it would rub on one of the washers. This uses the same hardware as the L-bracket mount (plus one square nut & screw) and provides support from both sides to prevent it from getting angled.

Print one of each piece, remove your existing L-bracket. Re-assemble the two pieces on either side of the bearing, 2x small washers, and 2x large washers with the same nut and screw as before. Discard extra washers.

To add an extra square nut into the rail, first, remove the plugs from the front face of the base. Loosen the corner brackets from the front extrusion rail. Slide it off. Remove the two screws holding the end-piece of the front X-axis guide shaft. Using a zip-tie or other small flat object, push the two nuts from that piece back so you can move one into position as the second nut for the idler mount. The front square nut of the two holding the X-axis guide becomes the back one. Add a second square nut into the channel for the front screw of that mount, reassemble. You can then attach the new X-axis idler mount using the original screw on one side, and a new M5 screw on the other. I only had a pan head screw, so I chose one a little longer since it didn't fit into the cap head socket I used for this model.

Sources: Cap head socket model and structural concept swiped shamelessly from the parallel z-axis bearing mount.

I first considered using the X-axis GT2 belt modification, but it required a lot more parts than I had (or wanted to get into right now).