I'm designing a menorah to build for next Channukah. Are there anything about the Law on how a menorah should look, other than that it holds 9 candles? Must it exactly resemble the traditional candleabrum, or may it also be abstract or even still-life art?
Any design which has all lamps at the same height and clearly separated from each other, and ideally in a straight line will work fine.
For a visual representation of the typical Oil Hanukiah this google image result is a good indication of a streotypical Hanukiah
For example at Chabad.org they say:
There are not very many requirements for a kosher menorah, and most—but not all—of the menorahs on the market are just fine. The basic elements of the menorah are eight candle (or oil) holders, and one more, that is set a bit higher or lower than the rest, for the shamash (attendant) candle.
That said, there are some factors to consider when purchasing a menorah:
The Chanukah lights can be either wax candles, or oil-fueled lights. Since the miracle of Chanukah transpired with olive oil—the little cruse of oil that lasted for eight days—the oil menorah is preferable to the candle one, and olive oil is the ideal fuel. Whenever purchasing a mitzvah article, we try to buy the most beautiful one that is within our means. So, if at all possible, go for the silver menorah. Beautifying a mitzvah is our way of expressing our appreciation to G‑d, and how dearly we hold His commandments. (A beautiful menorah also makes for a beautiful centerpiece for your silver closet or mantelpiece . . .)
The eight candles of the menorah must be arranged in a straight line, not set in a semicircle. Similarly, the menorah’s lights should be level or on an even slant, not some randomly higher than others.
Kitzur Shulchan Aruch rules that the eight substantive flames all be on a level with the helper flame higher; that the flames not be so close to one another as to form one flame (or to melt adjacent candles if using candles); that two wicks not protrude from the same opening of an oil lamp; and that [if using oil] the lamps not be earthen and used more than once. He has no other design restrictions, though he does mention a recommendation of silver as nice for those who can afford it.