How can I reduce a rough opening height 1-1/4"?
Cut a 2x4 to length and rip 1-1/4" off each side. Put those up against the top plate, one inside and one outside. The half inch hollow in between is inconsequential.

End view
 _____
|_____| &lt;-- top wall plate
|_| |_| &lt;-- 1-1/4" high rippers