put the hex in there and use a 2 part epoxy to glue it in place and fill the gaps. (it'll still be able to be pulled out later).
Worked for my 14mm RPM Wheels...(Metal Hex)
Maxx - FLM, MMM, CD Med.36x706s
Raja - FLM Ext. MM, 1512, 4s
SC8ight - MMM, 1520, 6s
D8T - MMM 1717 6s