Mine is open on either end, one side a little lower than the other, but I'm going to see about modifying it so they can get out the sides as well. I'd use my heating plate too but I only have one plug going out in the coop.Welcome!
So sorry for your losses, and I agree that open on all sides, and higher on one side than the other, works best. And I like to have plenty of space, so a larger brooder plate is also good.
Also, the chicks we've ordered from cackle, so they are shipped and stressed, are under a heat lamp for a couple of days, as recommended by the hatchery. We too lost a couple of chicks the first time we used a brooder plate, which was actually too hot for them until we adjusted it's temperature.
Mary