Here are some points to take.
1. The best support "beam" for an altar should be made of wood or plywood.
2. This is pure commonsense. If the standalone is a piece of glass; if the glass inadvertendly cracks; all the items on the altar would come crashing down. There is no issue if the base of the altar is a wooden plank and a cut-out piece of glass placed onto this wooden plank to make it easier to clean the surface of the altar is fine or ok.
3. Some recommend 49 3/4 inches from floor to the top of the altar.
Cheers.