Depending on how much money you have...
The cheapest way to build a grow box is to use several layers of cardboard/it`s porous so holds heat well,
Layer the inside with clingfilm and seal it with duct tape, as air tight as possible -except for the cracks round the lid,
The clingfilm allows you to retain any humidity built up in the box/it`s good for the plants up to a certain level.
Then over the top of the cling film I layered silver foil/as creases as possible and sealed with duct tape, the silver foil
reflects the light back onto the plants from all directions so as much of the plant can photosynthesis as possible.
I also cut a small hole in the side and put a computor fan in to circulate air,
This is a good way to build your first grow box, it`s relatively cheap but still effective,
Good Luck!