Try dimethyl carbonate + base (sodium methoxide in methanol should work).

Use the reaction of di- or triphosgene in a diluted ethyl acetate solution of diamine (1 g of diamine in cca 50 mL of EtOAc), temperature after an addition of di-,triphosgene cca 50 - 60 °C.

Another ways give 1,3-bis-urea only.

Have you tried ethyl chlorocarbonate (also named ethyl chloroformate), ethyl pyrocarbonate or carbonyldidimidazole? The latter is used in peptide synthesis where boc protecting groups are common.