... Anyone? ^^
Well that depends what do you really want to have.. an altar? A shelve? or just place to put things... like Buddha figure etc.
Altar has its meaning and also has properly organized space, levels etc. which should express so called shumidan
, i.e. a universe where Buddha is placed on highest point and everything has its order. You could see already a picture of the altar in this topic. There were no explanations of the meaning etc. however there are many meanings and the altar has its deep and profound meaning. A shelve etc. is used, but only additionaly for the guardians or local dieties. Nobody in Japanese zen house or other house would put there Buddha.
I have no idea what to do with any other altar... moreover an altar needs special consecration in the begining by trianed priest, also opening eye ceremony, and daily care which is form of practice because altar is the path and reminder of the 6 paramitas, which symbols it should contain, it should also express the 3 jewels and the outer form of buddhahood... so it is source of blessing, protection, etc.
But if you put somewhere things neetly in some sort of order and clean, you may consider it to be an altar. I have seen with my western friends many different things like this, and they called it ''altars'', which is fine, but it did not have any original meaning... but maybe it works for them, I do not know.