Here, I'm using openscad, to generate STL model.

The left side is, let me call it as upper one. At the conversion STL model to DWL66+ lithography, we need the selection either upper or lower. This selects the view point of model. Selecting upper will look the model from the upper, the lower looks from bottom.

// upper version
module slope (L, H, W, D, offset) {
    
     translate([-D * 2 , 0, 0])
         cube([D, W, H]);

     translate([D + 2 + L, 0, 0])
         cube([D, W, H]);

     polyhedron(
         points = [[   0 + offset, 0, 0],
                   [   0 + offset, W, 0],
                   [   L + offset, W, H], 
                   [   L + offset, 0, H], 

                   [   L + offset, 0, 0], 
                   [   L + offset, W, 0], 
],
         faces  = [[ 0, 1, 2, 3],
                   [ 3, 2, 5, 4],
                   [ 4, 5, 1, 0],
                   [ 0, 3, 4],
                   [ 1, 5, 2]]);
}

slope (100, 50, 100, 10, 0);

You need to set 'upper' at the STL import dialog at DWL66+ conversion software

The first one looks fine, but the lithography result does not necessary meet the intuition from the model. The second one here, say lower version, shape matches the lithography result.

// lower version
module slope (L, H, W, D, offset) {

translate([-D * 2 , 0, 0])
cube([D, W, H]);

translate([D * 2 + L, 0, 0])
cube([D, W, H]);


polyhedron (
points = [
       [ 0 + offset, 0, H],
       [ 0 + offset, W, H],
       [ L + offset, W, H],
       [ L + offset, 0, H],
  
       [ L + offset, 0, 0],
       [ L + offset, W, 0],
],

faces = [
      [ 0, 1, 2, 3],
      [ 3, 2, 5, 4],
      [ 4, 5, 1, 0],
      [ 0, 3, 4],
      [ 1, 5, 2]
      ]
      );
}
slope (100, 50, 100, 10, 0);

You need to set 'invert' at the STL import dialog at DWL66+ conversion software