One thing I like to keep on my altar is a bowl that holds meaning for me- I'm a potter so I generally use a bowl I've made- and I often keep it filled with water, and I sometimes add oils or herbs or flowers to the water. Perhaps something like that could work for your water side, if you have a bowl, cup or chalice that you like? Something simple and inexpensive.
For fire...maybe an incense holder on top of the wardrobe in that space with incense ready to burn, then when you're working with the altar you could light the incense? Or a tealight oil diffuser
? If you can't have anything burning there...maybe a small bundle of twigs to represent kindling? (Hey, Wands are usually thought of as fire, right?) Or alternatively, a fiery stone/crystal, like a citrine or sunstone?