Hi RR,
Well, the beer helps of course! When I fitted mine before opting for twin carbs I made sure that the angle between the carb and airbox was correct. You might or might not have a tufnel spacer between the carb and cylinder head? This could either help or be a hindrance to the fitting. If the rubber is prone to slipping off then I'd not use any lubricating liquid as such. If it is a good fit and is tight then maybe a slim type of jubilee clip on the carb end might hold the rubber in place while you push and prod to fit into the box. That's the best I can come up with. Good Luck!