First I bought a cage which had a bottom from some plastic. Then I went to the glass shop and they made an aquarium for this cage from 5 mm thick glass (so cage was first, then aquarium, not the opposite). 3 cm from the top edge they pasted two strips of glass as you can see on the drawing - the cage now stands on them.
My aquarium is under 30 cm high and degus don't have problems jumping up branches of the cage. If you want higher aquarium, put some branches or ladders there otherwise degus can't get up.