Fine, so let's do the maths on this.....
For £13 you can get a 5-in-1 "reflector" that will give you black, white, silver, gold and translucent materials, that is resilient and robust and packs away neatly, and will not break if a gust of wind catches it or you sit on it or lean on it. That price includes delivery to your front door.
By the way, a pedant might argue that a black surface is not a reflector either but I can see you're not here for an argument, so let's not argue.
Now, going the DIY route, you have to find a place that stocks foam core, and I haven't found any on the shelves of a UK based DIY chain, so you'll no doubt be driving around to find some, and at today's prices for fuel that could be £2-£5 quite easily, never mind the time it would take. So let's say you find some and it's £3, for example. B&Q do some spray adhesive for £3.98 and we'll say you can pinch the foil at home.
So your running total so far is around £10, plus your effort and all you'll have to show for your trouble is a fragile, unwieldy, reflector that only offers white and silver options. So if you do want gold and black, that's another £3 for some more foam core and more money for black and gold finishes, whatever they would be and whatever they would cost. That just leaves your diffusion material and a suitable frame across which to stretch it.
I will not make any comment as to which route offers the better value. You can decide for yourself.