you can elasticate a whip. Take the end section to your local tackle shop and try to fit a PTFE Bush on the end, one that will take up to a number 8 elastic. If it doesnt fit then cut a little bit of your tip section back until it does, I find a metal nail file best for this or a fine hacksaw will do. If the bush is too big then you'll have to use a larger one, possibly rated to a 14 elastic.
As for what elastic to put in, I suppose it depends what fish you are using it to catch. As a general rule a number 6 (usually yellow) will cope with most situations.
Lee Saville - Colmic Team Ossett
Individual 2002 World Club Champion, Zagreb - Croatia.
Individual 2002 Angling Times North East Region Winter League Champion.
2001 Embassy Division 1 National Team Champions.
I have to agree with Lee on the elastic,maybe even a soft eight elastic just for the extra bit of backbone.It also depends on the rating of your whip.Some dont have a rating,depending on the quality and what they are designed for.Andy