I know you can use the emergency foil blankets, Walmart ($2 US), a hardware store called Mac's also sells it right off the roll like fabric and other upholstery by the foot right off the roll, as an idea of places you might try to look around you. There is a special spray on glue that I used to use to adhere the space blanket to things like cardboard that worked really well, some brands are better than others for this. If you try to staple mylar make sure you use something like that cardboard strip from Weatherization Plastic for your windows you Nail on, that works great. I used to buy the plastic for my windows and then use it along side the mylar as a 2nd layer, roll the edges around the cardboard strip and then staple it up and as I was doing it for heat reflection and it worked out great. I would later learn about a mylar bubble wrap called "Reflectix" the hardware store Menards (cheapest I've found) carries it as does others like ACE and sold in the growing shops they really jack the price and call it IR Foil Wrap. I've also seen mylar on the roll sold in 25' to 100' rolls in places that sell growing equipment, that weren't overly expensive. The thing to know about Mylar is that it comes in different thicknesses and that is why some is more expensive than others.
I once did look at the cost to build your own tent, or buying a quality tent... the short of it was this, the cheaper tents have light leaks, some people complained the thread was breaking, zippers didn't work properly, some even had issues with the plastic corners needing to replaced where the polls connect, etc... All in all I just personally thought a cheap tent is just something to get started with and then hurry up and upgrade it hopefully before it falls apart. While the best tents seemed to be Orca Tents who makes the Ocra wrap, that stuff is seriously amazing, check out a YouTube video on a Mylar Tent compare to the Ocra Tent if you want to see the difference in reflective materials. Gorilla Tents made the best diamond mylar tents which were seriously strong, and meant to hold up to use and still be light tight, but you paid for it in the price too. In the end I choose Ocra and its more than paid for itself repeatedly because of the difference light reflection really does make.
