Sliding wardrobe fitting

[Tennentslager]

B&Q Spacepro brand. MIs say 2260 is the frame height, width 3m.

So my opening is 2270 at one side 2280 at the other.

How do I do this?

Was thinking a bit of 19mm planed timber attached to ceiling and plane it to remove the extra 10mm? Will it bend enough?

Thoughts welcome...

Floor is square, ceiling has been plastered after a leak and I reckon its the ceiling that's off-sloped.

Plan A is to cut a timber to 2260 then mark out the gap every 150mm or so across the 3m opening