I make those rods too. I typically use the collars/sleeves sold for making leaders as tips. You can buy them in different sizes to accomodate the tip of your rod.
Here’s another idea for you. I use shrink wrap I bought from Gander Mtn. to attach the spring bobber. It’s good stuff and comes in longer lengths that can be trimmed. It’s in the bowhunting section. It sure beats using the small section that comes with the spring bobber.
Could you please point my in the direction of a dealer that can get me 5 foot hollow rod blanks with .250″ butt? I can buy them @ Cabelas but they run me about $15. Anything cheaper would be great!