If it smells bad it’s time to put them into a bigger brooder. Birds grow SO fast. I like to use deep litter for them, but it doesn’t matter how deep the litter is if the brooder has become too small.
Pictures will help people advise you. For your numbers and their age, I suspect the largest available plastic tote (preferably clear) would be appropriate at this point. I cut a large window out of the lid and affix 1/2” hardware cloth in its place with washers and short screws. If you don’t plan to brood chicks often, you could get away with something makeshift. Just be SUPER careful of fire if you’re using a heat lamp. You need to cover it because they’ll soon be hopping out if you don’t. Also it keeps the warmth in. I lay some smaller tote lids over the wire loosely at night for warmth. (I use infrared brooder heaters or homemade heating pad brooder “caves”.)
Search brooding chicks outside for ideas about what you can do when they outgrow the tote. It won’t be long.