400mm f/5.6, cleaning rear element

So I picked up a used 400mm f/5.6 yesterday, and see some subborn dust on the rear element, but damned if I know how to reach that far down to clean it! I have a rocket blower but it couldn't shake the dust loose. I do have a lens pen, but it's too short. I had no idea that the rear element was so deep into the lens. My fingers don't even reach.

Anyone have any MacGuyver-type ideas to solve this dilemma?

I cleaned a sigma lens with a vacuum hose. Not saying it was a smart thing to do or not, but there was dust on the inside of the rear element and i carefully used my hand to control suction amount until there was just enough suction to clear out 3 stubborn pieces of dust.

I use a chopstick with its end wrapped in cotton to clean the recessed rear element of my 400 5.6L.