That's so amazing. What happy chicks they must be! I don't think I could do anything that fancy with the hinges and all but I definitely feel like getting a huge cardboard box and spicing it up! I've never heard of the mama heating pad. Do they not need constant warmth on them? I thought they did, thus the heating lamp.
Cuddling them is so much fun, they really like to snooze in
I had a similar brooder last year out of a slightly smaller cardboard box and using the same door, and I used zip ties instead of hinges. It worked fine. This year I decided to go fancy and give the front a wooden frame with real hinges

So you can definitely get away with just tape and zip ties!
The mama heating pad method approximates what they would get if they were with a mama hen, so it's more natural and better for them for many reasons. They do not need heat on them at all times. If they were with a hen, they'd go under her periodically to warm up, then come right back out and explore the world. Even in colder temperatures. I have a broody with chicks right now, same age as my brooder chicks, and she hatched hers in a snowstorm in April (snowstorms in April do happen over here in MA). For their first week, nightly temperatures still dipped below freezing, and daytime temperatures were in the 40s and 50s, but the chicks were outside with their mom walking around being fine, going under her only occasionally to warm up. So that's what the heating pad provides. They tuck under it as they would under a hen, and know what to do. Chicks have a VERY strong instinct to tuck under something soft for warmth, and if they don't have a hen or a heating cave like that, you might see them trying to go under each other or under a stuffed animal if you gave them one. So the heating pad takes advantage of that need. It also hardens them off better, since they aren't being heated all the time. They learn to regulate their body temperature better, and can control their own weaning off of heat when they start growing up. You don't have to keep measuring the temperature and wondering when and how you need to change it as they grow. They do all that themselves. And, since the pad gives them heat but no light, unlike the lamp, they get to have a real day/night cycle, which they need. There's no lamp glaring down on them 24/7. Chicks with a light chirp more and are more agitated, including trough the night. Chicks with a heating pad go to sleep at sundown and wake up at sunrise and sleep motionless and perfectly quiet all night, not a single peep to be heard. And last but not least, a heating pad is better than a lamp because it's not a fire hazard the way the lamp is. So, the heating pad system is awesome and I highly recommend it! You can get a cheap heating pad from
Amazon (just make sure it doesn't have an auto off). I have mine bungied to the underside of a wooden board. Then for legs I have long bolts with nuts, because that way you can raise it as they grow. They need to be able to touch their backs to the heating pad, because it transfers heat via direct contact, it doesn't heat up the air.