There is a melting point experiment I did in a college chemistry lab that involved melting glass tubing just to the point that we could stretch it to create a much thinner tube. Glass (depending on type) melts at a much higher temperature than what you’re using here for splicing, so that might be an easier and more reusable method for you?

Edit: search for “glass delivery tube”, should get you to what we used. Make sure it’s borosilicate and not polycarbonate