Take a short length of PVC pipe, the diameter of which is just large enough to slide over the telescopic whip of your portable. Wrap several turns of your antenna wire around the PVC pipe. You will need to secure the wire to the PVC with electrical tape or something. Epoxy would be a good, permanent way to secure it, but would be messy to handle until it hardens. The PVC pipe I used had two holes in one end that I put the wire through before wrapping it. The inner diameter of the PVC pipe was large enough for the wire to fit with the telescopic whip inserted. At the other end of the wire antenna, I attached a large fishing weight so I can throw the antenna up into a tree. An alligator or other clip could be used to attach the wire to a curtain or something for indoor use.
The length of the wire antenna and PVC pipe aren't critical, nor are the number of turns of wire around the pipe. However, the more turns of wire around the PVC or longer the wire will increase gain, and too long an antenna may start to overload the receiver. Ideally the wire should be grounded on the receiver end after passing around the pipe, but I've found that this really isn't necessary.
A similar design has the wire wrapped around a toroid "donut" which then slides over the whip. This is also an effective way to couple the antenna to the radio.