Hello @Reginald_Williams. Have you considered creating your text inserts with Photoshop, Gimp or Photopea (online free clone of Photoshop) ?
I suggested it to someone else here : About add shadow in letters
You can even make your black background there if you want to. Two birds with one stone. Also, if you use a screen shot from Shotcut, it would make it very easy to position your text and black background on the screen.