You obviously drew a rounded rectangle, applied -30 degrees vertical skew and then it didn't fit.
The clou is to have right dimensions before skewing:
The black rectangle has width = 3 grid horizontal diagonals. For testing other things in the same image there's also equally wide red rectangle with sharp corners. Those other things:
- removing the rounding from the bottom corners and
- adding some thickness
Transformation Skew 0 degrees horizontal, -30 degrees vertical is applied.The black rectangle fits exactly to the grid. The shorter side is now 6 grid units (=2*3). The red rectangle is skewed, too
3.-4. Red rectangle is subtracted from the black rectangle (Path > Difference) to remove 2 roundings easily. As well you can make an Union. The latter can save the height intact. Here the original black rectangle shortened one grid unit,
The shape is duplicated and moved a little to up and to right. You must switch snap to grid and snap to points (you need them, too) on and off in turns. With high zoom 3 sub-grid units shift snapped easily. Then
- draw a diagonal (=the red line)
- Draw 2 lines (=cyan) in parallel with the grid lines. They become the corners. The sharp corner at bottom right is drawn directly to its final place.
Place the corner lines. You need point snapping.
- move the beginning of the upper corner line to the crossing of the rectangle and the red diagonal
- rise the further black rectangle on top
- with the node tool insert a node to the further rectangle at the crossing with the cyan line
- select the new node and the old corner node (see the orange arrows) and apply "Break path at selected nodes" in the info panel NOTE1: The rectangle is now a combined path. It must be broken apart before trying to delete the invisible part.
Use Path > Break apart and DEL to remove the invisible part of the further rectangle. Delete the diagonal. Shorten the corner lines to fit. Make them black. There's no need to join the pieces if you use round stroke ends.
NOTE2: The following shape was created in a freeware 3D program. It took a minute. The shape is available as editable 2D vector drawing if one prints the scene as PDF and opens it in Illustrator or Inkscape.
The latter needs a trick. Filename extension pdf must be changed to ai. Opening in Illustrator is the better option, if available, because Illustrator can automatically simplify PDF's multilevel groups. Opened in Inkscape this shape had 20 level grouping - like every new element was grouped with the group of already inserted elements.
ADD: Inkscape's extensions menu fortunately has "Arrange > Deep Ungroup" which is useful in this case. It flattens multilevel groupings in few seconds.
Used freeware CAD = DesignSpark Mechanical, a heavily decimated version of SpaceClaim, but still useful.