Same here except I too did it a bit differently. I removed the bed platform hinges so I could then pull the larger platform forward 6" and add a 6" wide piece of same thickness plywood to give me a full 78" bed.
Now that I had a longer platform I simply pulled the existing short queen mattress forward, creating a gap at the "head" end of the bed.
I then installed a 3" gel memory foam topper that turned a rock hard spring mattress into luxurious comfort. Since the topper was a full 80" long I used the excess to fill the gap at the head end of the bed.
Adding a second piece of plywood under the "foot" end of the bed provided additional strength so I didn't need to add legs to support the weight of anyone sitting on the end of the bed.