Depends on how much precision you qualify as necessary. For a 45 degree isometric it's relatively easy, you'd only have to calculate the short sides of a triangulation. A 60/30 isocube needs some additional calculations; being lazy I'd definately fire up some CAD software to produce a nice SVG cube.

A 45 degree isocube with dimensions of 5.00cm; draw a rectangle (press Shift for equal dimensions), right click --> Position and Size; give width and height as 5.00cm. Apply. Then go to Edit --> Duplicate and choose 1 copy. Now, right-click on the copy and select Position and Size again; go to the Slant and Corner Radius tab, set slant angle as 45 degrees.

Then in the Position and Size tab; set Size : Width as 3,535533906cm which is the Pythagorean value I calculated; √(5.00²/2). Draw will round the number up to 3.54, please use more accurate units than centimeters, this is only for demonstration.

The resulting polygon can be duplicated, flipped and rotated to construct the cube, alignment to gridpoints helps (at least with such simple dimensions).

Maybe this is possible some other way I'm not aware of, already tried to manipulate the included shapes in Draw without much luck (precision-wise that is).