Fletch,
You cant make a custom room unless you are a subscriber! But any subscriber can make a custom arena and then you can play for free,all you need is an account but you dont need to subscribe!
I'm usually in the training arena most nights,there is no lack of training as some might think but there is a lack of players wanting training! I spent about an hour with a couple of players the other night,I show one of them a simple concept and a light went on for that player! All he needs do is work on it and pass some on to others in his squad. When this happens it gives me great satisfaction and the player often is successful afterwards.
Those who think the learning curve is steep,are wrong,it's more a series of steps and plateaus! Learn a new skill,become proficient then on to the next skill,it almost never ends because of the diverse game play,large amount of players and the large play area that AH3 offers!
YMMV!