Here is an example, use this code and modify the dimensions.
% Define the first square of size 1
x1 = [0; 1; 1; 0]; % x-coordinates
y1 = [0; 0; 1; 1]; % y-coordinates
% Define the second square of size 0.5
x2 = [0.25; 0.75; 0.75; 0.25]; % x-coordinates
y2 = [0.25; 0.25; 0.75; 0.75]; % y-coordinates
% Concatenate coordinates for decsg input
rect1 = [3; 4; x1; y1]; % First square definition
rect2 = [3; 4; x2; y2]; % Second square definition
% Create geometry matrix for decsg
gdm = [rect1, rect2]; % Geometry description matrix
ns = char('R1', 'R2'); % Name space matrix
sf = 'R1+R2'; % Set formula
ns = ns';
% Create the 2-D geometry using decsg
gd = decsg(gdm, sf, ns);
% Create a fegeometry
gm = fegeometry(gd);
% Now, extrude the 2-D geometry to a 3-D geometry with a length of 1
gm = extrude(gm, 1);
% Merge the cells in the 3-D geometry
cellIDsToMerge = [1, 2];
gm = mergeCells(gm, cellIDsToMerge);
% Find the smaller of the two faces at the top
% We use a point above the center of the smaller square along the z-axis
topSmallerSquareCenterPoint = [0.5, 0.5, 1]; % Row vector for coordinates
topSmallerFaceID = nearestFace(gm, topSmallerSquareCenterPoint);
% Extrude the smaller top face by a length of 1
gm = extrude(gm, topSmallerFaceID, 1);
% Rotate the complete geometry by 180 degrees around the x-axis
% to place the new smaller cube at the bottom
axisOfRotation = [1, 0, 0]; % Rotation axis (x-axis)
pointOnAxis = [0, 0, 0]; % A point on the axis of rotation (origin)
angleOfRotation = 180; % Rotation angle in radians (180 degrees)
gm = rotate(gm, angleOfRotation, axisOfRotation, pointOnAxis);
% Now the complete geometry is rotated, and the smaller cube is at the bottom
pdegplot(gm)
