While adding plastic mesh hardware cloth on all sides and the bottom of the cage will make it safe for your gliders, it will also make it a huge chore to clean. Every point where the mesh meets the cage bars is a spot that will accumulate glider pee and food waste where it will dry and be impossible to just wipe away.
You will need to attach the plastic mesh at numerous points all over the cage because the gliders might go between the bars and get stuck between the bars and the mesh unless it is very tight.
I suggest selling the cage to a large bird owner and purchasing a large glider safe cage in its place.