Two weeks ago I took a 5mm soda-lime glass piece and spread some aquarium 100% silicone sealant on it.
2 pieces - 5mm 150mmx150mm ordinary glass
2 pieces - 10mm 110mmx70mm ordinary glass
1 tube of 100% Aquarium silicone.
1 piece of heat shrink wrap large enough to cover 150mmx150mm and have a margin of about 3cm on all sides.
Spread silicone and cover with shrink wrap place the other 150mmx150mm glass on top and use the 10mm to spread the load and clamp.
If you get bubbles peel and start again. Detailed instructions available at the ChemShapes site HERE
Leave untouched for 2 weeks.
Today - Carefully removed the 10mm glass and clamps. Clamped on one of the 10mm pieces for a few hours to make sure silicone has cured - only to check.
Guess not. But a few hours later I managed to peel off the shrink wrap from the silicone and left it to set.
Some things take time, so prepare and experiment early on to be able to have something useful at the right time.