Just a thought as I work away a lot and struggle to get on sometimes but I will always try donate .
heres my thought i think you should be able to once your barraks are full u can click on the troops u would like to donate IE I always train 200 level 7 archers to donate but it's hard to be on all the time so a auto donate button were u can select would be good for everyone I think.

thoughs would be good

cheers ben