Use Inkscapes box tool to draw out your basic cube. The sides may have just a fill so turn off the fill and add a stroke. You should have the skeleton of a 3D box.
The box is drawn in strokes so let’s convert them to paths (see the path nenu).
Now you can ‘simplify’ the path (also path menu) and sculpt nodes by hand to roughen and round the corners and edges of your cube to make it more ice-cubey. I didn’t add highlights to the faces of my ice cubes but you can.
Your 3D box lines all have the same opacity, which is fine for super-simple illustration, but we might like to make the edges on the rear faces of the cube appear less opaque as if you were seeing them through the ice. I did this with a semi-opaque mask that I drew over them with the path tool.