Ex-head of state Khieu Samphan, chief ideologue Nuon Chea face genocide charges at the trial that begins Wednesday in Phnom Penh.
Chum Mey sells books about the Khmer Rouge, and about himself, in what is now called the Tuol Sleng Genocide Museum.
The UN-backed Khmer Rouge tribunal has faced growing criticism in recent years for failure to provide justice to victims of the regime.
At Thursday’s ceremony, Khmer Rouge survivors Chum Mey and Bou Meng both said they were happy a memorial was going forward.
Operations at S-21, the brutal Khmer Rouge security center that Duch ran, will be included in the upcoming trial of Nuon Chea and Khieu Samphan.
The memorial Buddhist stupa will be built on the grounds of the former center of Tuol Sleng, which is now a war crimes museum.
The health of aging Khmer Rouge leaders is of ongoing concern, as many fear they will not face justice for crimes committed by the regime.
The UN-backed Khmer Rouge tribunal will hold an initial hearing for two aging regime leaders July 30, in anticipation of the beginning of a full trial at the end of the year.
Opposition officials say they were prevented from meeting in the former Khmer Rouge stronghold of Anlong Veng by a group of soldiers clad as civilians.
Ieng Thirith was one of four original Khmer Rouge leaders arrested and expected to face trial for atrocity crimes at the UN-backed tribunal.
Officials at the Victims Support Section said in a statement Wednesday that they want to hear from lawyers and victims, as well as donors, as they discuss the upcoming trial phase.
The survey, conducted by the Cambodian Defenders Project, found that some 60 percent of 105 surveyed respondents experienced some form of sexual violence under the Khmer Rouge.