No point getting light to a plant that hasn,t sprouted , when its under the soil its dark day and night .

I don,t know what shops you have in your country but you need a the sort of shop that you would go to if you where doing home improvements.
Either get thin board which is white on one side or just buy some undercoat (primer) this will be flat wight, It comes in emulsion or oil based.
The emulsion is water based and is quick drying and nicer to use with less mess.
Disco.