IT is one of the simplest means of trafficking drugs into Hong Kong, but the nasty aftermath is literally making Kai Tak Customs officers sick. Drugs couriers come in all shapes and sizes. And a growing number of mostly Nepalese and Nigerians flying into Hong Kong conceal their illicit stash in condoms and swallow them before starting their journey. In the pit of a stomach, the prophylactics and other paraphernalia, typically filled with heroin or cannabis resin, shows up as a faint outline on the screens of Kai Tak X-ray machines. But then Customs officers must start the distasteful process of retrieving the suspected narcotics. ''And this can only practically be achieved by a bodily function over a bed pan with an officer on hand to ensure the suspect does not attempt to destroy, flush or tamper with the incriminating evidence,'' a source said yesterday. But the dirtiest job in the business is set to be revolutionised by the acquisition of a $100,000 custom-made ''drug loo'' for Hong Kong Customs. The ''drug loo'', produced and marketed to law enforcement agencies the world over by a company in the United Kingdom, performs the odious task which hapless Customs officers presently have to do. Its special design allows for the filtration and maintenance of narcotics discharged by drug couriers. ''The basic function of the product is to provide operating officers with a socially acceptable means of dealing with packets of drugs or other illegal substances concealed internally,'' said Adrian Wirth, a company official. Hong Kong Customs officers have studied the ''drug loo'' and believe that it would make life a lot easier at the new Chek Lap Kok airport for staff, who complain of feeling nauseous while supervising the nasty job, the source said. More than 400 people arriving in Hong Kong were asked to undertake a test for concealed drugs last year, resulting in about 20 seizures. ''It is incredible, but some couriers like the big Nigerians coming to Hong Kong can swallow over 100 condoms, each one containing about 10 grams of heroin,'' said a Kai Tak official. A Hong Kong-based United States agent said passengers who would not eat or drink anything during their flights aroused the suspicions of authorities at international airports in the US.