I've never done GUIs so this is a guess...
From subplot doc's
subplot(..., PROP1, VALUE1, PROP2, VALUE2, ...) sets the specified property-value pairs on the subplot axes. To add the subplot to a specific figure pass the figure handle as the value for the 'Parent' property
Or, if that doesn't bring joy you can always use the form to place the axes precisely where you want them and at a given size...
subplot('position',[left bottom width height]) creates an axes at the specified position in normalized coordinates (in in the range from 0.0 to 1.0)
The latter would be handiest to use if you were to encapsulate it in a helper function, probably.