Honda XL Italia

Hi,
I removed the plastic air filter case last year for a restoration but can't get it back in again.
I seem to remember removing the rear wheel and sliding it out backwards.
I've tried putting it back this way but it's too wide to get through the frame.
Is there a trick to this?

Here's how it's done, courtesy of the German XL site.

I only needed to remove the rear wheel and shock absorber, leaving the suspension linkage and carburetor in place.....

STEP 1
Slide the air box in from the rear of the frame, and with the right hand side at a slight angle so that the step for the battery passes around it first.

STEP 2
With the battery step now around the frame tube, straighten up the box and it should slide forwards and into place.