The conference will take place in 2026 after being cancelled for a year in light of the Supreme Court's gender ruling.
The group Take Back Power says its members poured custard and apple crumble on the display.
The former Duchess was an "ambassador" for vVoosh, founded by her friend Manuel Fernandez.
Heavy rain and strong winds are forecast for the start of the week.
Malcolm Offord appeared at a rally alongside Nigel Farage in Falkirk on Saturday to announce the move.