hi BIG DAVE
try puting them together off box first lay cross members out
then next you will have 4 brackets with 6 holes in each these
fit on xmembers 2 on each you will have 4 screws of 1 size 1 for each bracket, these can be fitted & tightened.
then dependending which model you got there should be 4 spacers with 4 plain holes in each & 4 spacers with 4 holes & inlets for nuts .the first set i mentioned go between xmember brackets & box on out side the others go on inside.
Ive always found it best to drill one hole first with 1 side in position fit 1 bolt then drill other holes in that side & fit the rest of bolts then repeat with other side