Think of an IBM Travelstar 5GB
It's totally inexpancive(5GB is pretty OLD product you can buy for few bucks) it's not that much bigger than a PCMCIA card. Then you buy a slim USB external box for notebook HDD(auto-powered by USB), put in it the travelstar, then buy USB host CF card, install storage driver for PPC and you got portable 5GB for 1/10 the price(or more!) of 4GB CF card. Yes you need extra battery and isn't small as a CF but it works fine!! Really this solution is not that big/uncomfortable than PCMCIA jacket(expecially dual pccard!)with PCMCIA HDD.