The sipper tubes listed don't fit into the rubber stoppers that...

The metal sipper tubes fit snugly into the rubber stoppers. This is necessary to avoid leaks. You'll have to use significant force.

We normally use a couple of wooden blocks, one with a hole drilled slightly larger than the tube. Place the stopper over the hole. The other block is used to place even downward pressure on the tube as it goes down into the stopper. The great thing is once the tube is in place, you don't have to take it out again.

Alternatively, you could use a natural oil as lubricant.