Maybe not an answer 
 
I think that `Inset[]` is not what you need. 
 
 Look what happens when you resize a graphic with some `Inset[]` : 
 
[![enter image description here][1]][1]

What you need is rather a layout that doesn't change when resizing.


Like this : 

 height = 125;
 cb = MatrixPlot[
 Table[Mod[i + j, 2], {i, 1, 2}, {j, 1, 8}]
 , ColorFunction -> "BlueGreenYellow"
 , ImageSize -> {Automatic, 2*height}
 ]; 
 
 p1 = MatrixPlot[
 Table[1, {i, 1, 1}, {j, 1, 2}]
 , Mesh -> True
 , ColorFunction -> ColorData["DarkRainbow"]
 , ImageSize -> {Automatic, height}
 ];
 
 Graphics[{cb[[1, {1, 2}]], Translate[p1[[1, {1, 2}]], {0, 2}]}, Frame -> True] 
 
[![enter image description here][2]][2]


 [1]: https://i.sstatic.net/tiDn0.gif
 [2]: https://i.sstatic.net/0tPts.gif