To make the platform for the stage I used loft board. I used 7 boards measuring 13'' each. The loft boards worked really well as they are strong, durable and they can interlock.
Firstly lay as many loft boards as you need on to the stage. You may need to cut them a bit shorter if there are any wall sockets. Once they are all in position screw them in with 40mm screws.
After all of that is done you can paint it, I chose to paint mine white!