Try Used glass wool of thickness of 3–4 mm.

Flat glass is easily available one but dome shape store more heat inside the solar pond

Use low iron contain glass available in market. Also, if you use dome shape glass cover, you may increase cosine losses with incoming solar radiation. Go with flat glass as it will be cheaper and easy to use. 

Good idea Atul Jadhav

Thanks for ur support

I think the dome shaped glass will withstand larger pressure. Also, anti reflective coating may help to harvest more solar radiation. The thickness of the glass depends on the required temperature gradient across glass walls. As  thickness increases the the thermal isolation increases. But one can use double wall glass with vacuum between the double sheet. In this way one can save glass and get more thermal insulation.

Wish you success