making a growing box

i said to the wife the other day why don't we grow our own petunia's instead of buying in she said we hadn't had very good results from growing our own this year so I looked into growing petunia a bit deeper where we mite be going wrong

and I was told petunia needs  good bottom heat and plenty of light to get good germination so I thought it mite be a good idea to make a grow box got the gist of how to make it when I went on line so I've decided to make one using heat cables in sand with a florescent light installed in the roof of the box no quite sure if I should line the box with aluminium foil for heat or poly sheeting painted white for maximum light?


anyway its going to be suck it and see either it works or continue to buy our petunia's in