I have the same lance (bought from Ebay) and I am using quite an small low powered Kecher pressure washer with Magifoam. This works very well together and you only need 1-2" in the bottle with some hot/warm tap water mixed.

IT IS VERY IMPORTANT THAT YOU ENSURE THE INSIDE OF YOUR BOTTLE REMAINS BUBBLE FREE. A FEW CM OF BUBBLES IN THE BOTTLE IS OK AND WONT CAUSE YOU TOO MUCH ISSUE BUT IF YOU LET THE INSIDE OF THE BOTTLE BUBBLE UP YOU WILL ONLY GET A VERY THIN SPRAY OF FOAM. IT TOOK ME A WHOLE 5L OF MADIFOAM TO FIGURE THIS OUT! KEEP AN EYE OUT NEXT TIME AND IF YOU ARE NOT GETTING A NICE THICK FOAM OUTPUT THEN IT WILL BE BECAUSE YOUR BOTTLE IS FULL OF BUBBLES AND THE LANCE IS NOT ABLE TO DRAW IN ANY AIR TO MIX THE SOLUTION.