I am just a layperson and uneducated at that but there appear no responses for quite a while so I will venture one.
The teacher buddha is usually to my observation the focal point or center. There is however to my knowledge any strict prohibition against other considered equilivent enlightened persons in the tibetan tradition being sole issue. Most commonly the teacher buddha is at the center or some representation of him which may not necessarily be a statue. A picture of something to remind us of his nature may be all that is required.
Others would be to the side, not lesser but equal if they are equal. If I had the teacher buddha in a alter I would not have him lower than another.
Offerings of a symbolic sort would be to the front.
Water being most common in a daily practice to my observation. Quite a few bowls and each filled in a certain way and direction. But the internet shouild supply the specifics.
I frequent wilderness at times and cannot carry a alter with me. As such a sparkling light in a speck of dew or raindrop piece of sap may reflect a rainbow may serve as my alter.
If you use such symbols such as picture or statue I would consider them as real. Don't treat them with disrespect walk over them or put things in front directly of their faces. When filled statues are moved or carried closely one may consider blindfolding the statue. A filled statue is pretty much like having a real person in that place so I would conduct myself accordingly. NOnfilled...a symbol or representation but still as intended holy to be treated in a holy manner.
So that's my obervation. Others may provide definitive answers of better quality.
Other traditions to my experience...... this varies greatly. I have heard of some traditions that will set a altar with a purposely empty place in center to emphasize empty nature. So I'd say it varies greatly.
It's to a degree with what you have. If you have no money or circumstance like I say a drop of dew may do
Any altar may be better than none. Mailirepa seemingly did not lug around altars in his places of retreat. So I wouldn't worry. What ever you do is bound to be good as long as you don't inadvertantly disrespect things by unthinking actions.
"This order considers that progress can be achieved more rapidly during a single month of self-transformation through terrifying conditions in rough terrain and in "the abode of harmful forces" than through meditating for a period of three years in towns and monasteries"....Takpo Tashi Namgyal.