For a simple "sandwich" model composed of two roughly flat and parallell profiled sides connected by a ribbon around their boundary, it's easy enough if you can triangulate one of the flat sides.
% some parameters
d = [4 1 2]; % block dimensions [x y z]
s = [1 1]; % slot dimensions [x z]
os = 1; % slot offset on x
% vertices and faces for the front side
V = [0 0 0; d(1) 0 0; d(1) 0 d(3); s(1)+os 0 d(3); ...
s(1)+os 0 d(3)-s(2); os 0 d(3)-s(2); os 0 d(3); 0 0 d(3)];
F = [6 7 8; 1 6 8; 1 2 6; 6 2 5; 2 3 5; 3 4 5];
% create the back side faces and vertices
nverts = size(V,1);
V = [V; V]; % duplicate (x and z stay the same)
V(nverts+1:end,2) = d(2); % offset y
F = [F; fliplr(F) + nverts]; % flip the winding and offset the indices
% since our object is two parallel faces with the same number of
% boundary vertices, we can just stitch those together (lofting).
% our boundary vertex list is simply sequential
va = (1:nverts).'; % front
vb = va + nverts; % back
vc = circshift(va,-1); % front
vd = vc + nverts; % back
F = [F; va vb vc; vd vc vb]; % add them to the list
% write to file
stlwrite(triangulation(F,V),'testfile.stl')
% display it using patch()
patch('faces',F,'vertices',V, ...
'facecolor','w','edgecolor','k');
view(3); camlight; view(10,33)
axis equal; grid on
xlabel('X'); ylabel('Y'); zlabel('Z')
If you're asking how to do CSG on an existing model, that's something else.
