Here's an idea for anyone struggling with the double bed in their 17-58 . I got this mattress and infill made so it is now queen size. I have a piece of ply underneath that slides out to support the overhang.
G'day mate the mattress cost $450 from The Bed Shop in Ballarat that included the little infill bit. Just a note to all I have had to send the mattress back as I had a bit of an oversight. I found the mattress hits right on the door handle when extended making it hard to close and lock the door from the inside so am getting about 150mm cut off the corner on a 45 degree. Will post pics when I get it back.