practical is to create a vertical carrier off a back rack using suitable, foam lined PVC tubing of a suitable length and diameter and create a scabbard. also, like some have suggested too, strapping long items along the top tube.

you can usually just SHOVE things between panniers and racks, or simply strap an item vertically to a rack with stout strapping methods like a few toe straps.

I don't have a picture of a PVC 'ski bucket' on a bicycle , but here's some skis strapped horizontally to a top tube,

then vertically to a touring bike. stable up to 40 MPH, you can do the same w a training sword.skicarry2.jpgskisgoingup.jpg