With Canva's frame functionality, you can only frame one item at a time, which can be very limiting. So, I've created these "reverse-mask" cutout frames. Rather than dropping an item into the frame like you do with Canva's frames, you'll just overlap your whole multi-element scene with one of my cutout frames. It creates a "window" into the scene.
 
These are in vector format so the color can be changed and they can be scaled to any size. Each frame is in its own SVG file.