Ohhhh this is fun- there are so many ways you could go about representing the elements! I absolutely agree with Amethyst that the glass could represent both earth (sand) and fire (fire needed to form the glass).
I think you will find that you have lots of options with finding things that represent multiple elements (it’s a fun activity to look around and examine what things contain which elements) As most items have been created from using different elements, I actually think it’s harder to represent pure forms of each element.
For representing elements, here are few additional options to consider:
ALL: Symbols or Sigils- although not necessarily made of the element itself, a magickal symbol or sigil is an easy and neat way to draw on the power of each element
ALL: Crystals- crystals are formed over many years by each of the elements: earth shaped by heat, pressure, and other natural influences. Additionally, every crystal has elemental correspondences.
Earth: rocks, crystals, dirt, plants, wood
As earth is the most stable element, earth items will likely be the easiest to find and maintain
Water: water (kept in a cleansed bowl or vessel), ice, plants (made up of mostly water), sea glass, river stones, sea shells
Air/Wind: air, feathers, empty space, dried leaves
Wind/Air can be the hardest or easiest element depending on how you go about it, as air is a constant presence- always there but hard to contain
Fire: candles, lava rocks, glass, charcoal, ash
While traces of fire energy are easy to come by, keeping the element itself can be difficult and dangerous
Just a few suggestions! I’m sure whatever you choose to represent the elements will be perfect. Good luck and have fun developing your sacred space, Tem!