You sir, must be referring to the BOOMERANG... slots nicely into the hop up brackets to close in that gap. Then insert two/three 600mm battens, or cut a size in there.
If you can not get them.. well I have seen all sorts of attempts to make something up... ermm would perhaps suggest just banging in a 600 ledger off one of the cups on a bracket to act as a transom, then lapping boards from the other side onto the 600 and clip the outside one down.. will look **** though. Otherwise handrail off around that gap. If you have three inside boards, use a 900 but then you have to spur up somehow and it starts to look like a mess.