private Component buildFileBytesCombo() {
		Memory memory = model.getProgram().getMemory();
		List<FileBytes> allFileBytes = memory.getAllFileBytes();
		FileBytes[] fileBytes = allFileBytes.toArray(new FileBytes[allFileBytes.size()]);

		fileBytesComboBox = new GhidraComboBox<>(fileBytes) {
			public Dimension getPreferredSize() {
				Dimension preferredSize = super.getPreferredSize();
				preferredSize.width = 100;
				return preferredSize;
			}
		};
		fileBytesComboBox.getAccessibleContext().setAccessibleName("Byte Source");
		fileBytesComboBox.addItemListener(e -> fileBytesChanged());
		if (!allFileBytes.isEmpty()) {
			model.setFileBytes(allFileBytes.get(0));
		}
		return fileBytesComboBox;
	}
Define the Main panel for the dialog here.
Ghidra/Features/Base/src/main/java/ghidra/app/plugin/core/memory/AddBlockDialog.java