I used to do the same trick with a 3-4 inch piece of drinking straw,(I recommend a Wendy's or Rally's straw,they're slightly bigger in diameter than most straws) and a length of insulated wire from a phone cord,enough to give you 18-20 turns on the straw and a 7-8 inch pigtail to hook to your longwire antenna w/ an alligator clip. You slide the straw up and down the antenna to find the "sweet spot" for best reception,ie; coupling w/ the internal ferrite antenna.
It does a nice job on radios that don't have an antenna jack or ones that do but overload w/ more than a few feet of wire.
To soup it up you can use the tuning capacitor from a junked radio between the straw's pigtail and the longwire. It acts like a combo antenna trimmer and frequency trap. When you've got it set up this way,you can move the whip away and back towards the radio for phasing,knocking out a lot of qrm and giving your longwire some directionality. When you're hearing what you want you peak it up w/ the capacitor for best listening.