In the parts manual it is called a heater valance assembly. As previously stated, I have both parts, LH and RH, but they are “plain” with no brackets nor attachment holes or securing devices.
It appears that a bracket was used but I am uncertain by looking at the parts diagram.
The 'valances" are held in place by finish screws and washers that attach to the adjoining trim by sheet metal screw capture plates. The one exception is the front lower corner of the valance which srews directly into the fiberglass with finish screw and washer.
I have a Elan S3 coupe and the way I attach my valances was to use double sided trim tape that is use to hold exterior trim to the outside of cars. I used it along the bottom of the dash and used a screw with washer into fiberglass covering the heater stuff on each side. Be careful as the valance is breakable. I do not know how your +2 attaches but that is how I attached mine.
Sarto