I don't know why you lost those two. Trampling doesn't make sense to me. Trampling usually comes when you have a lot of chicks and something frightens them. They panic and pile up as far away from whatever scared them as they can get. It can happen against a wall but getting trapped in a corner is more dangerous. Besides, it was two of your largest. Usually the smaller ones are more at risk.
When they are being raised by a broody hen they sleep in a group under her. In a brooder without a broody hen they still sleep in a bunch, I think they just like the company. It does not mean they are cold.
What is your forecast low at night, not your high during the day? That's what will be important. I think the best brooders are those that keep one spot warm enough in the coolest temperatures and a spot cool enough in the warmest temperatures. I don't know what that area looks like but I'd want the far end to be ambient if you provide any heat. I'd provide no heat during the day and I'd use a low wattage bulb at night if any.
During a heat wave a few years back I stopped daytime heat at 2 days and nighttime heat at 5 days. They didn't need any heat.