Step 1: Cut the jug in half..
The first step is to very carefully cut the jug diagonally - from one corner across to the other. I used a utility knife - be careful. Cut as straight a line as you can. Cutting through the threaded top will offer more resistance.
Step 2: Cut the access hole..
Next pick the half of the jug without the handle and cut an 'X' in the side to insert the hose extension wand. You don't want the wand sticking through so far that it hits the floor though. Then use duct tape or wide masking tape to secure it.
Step 3: Make a cut out in the front edge to suck up the debris..
If you stop at this point when you put the jug agaist the floor it will just act as a giant suction cup. You need to cut an opening along the front edge to make an entrance for air and debris. Also tape the threaded top where the cap screwed on to seal it against leakage. You could make all sorts of other specialized shapes this way too - for instance dust hoods for wood working machines, fume extractors, etc.