The classic solution is either a XL Rubbermaid plastic box or a XL Colman cooler. In both, cut a slot for the B&C to stick out. Don't understand why Husqvarna doesn't make a XL power box.
There have been several threads on this. Plywood works, but gets heavy, quick.
Several people have modified PowerBox-type cases (generic version is cheaper) to fit various saws. This can work if you only need a little extra clearance, or need a handle to protrude. Cut out recesses or holes with a sharp utility knife or Dremel tool, then reinforce cut sections by foaming the cavity or with hardboard ('Masonite') and Pop-Rivets.
If you use your saw intermittently and are looking for a
storage box, we found that removing the bars from large saws makes storage options much easier (one of those obvious in hindsight things).
A few of my creations. The cooler was done for fun, right after reading someone suggest it in one of these threads, when I walked outside and saw the church next door having a rummage sale. Someone else (?) modified a PowerBox by just letting the top handle poke through the top and side panels.
Philbert
Rummage Sale Cooler Case
PowerBox Modified for Extra Bar
Generic PowerBox Modified for Charger, Chain Adjustment Knob, etc.