I got tired of the kids dumping white sand all over my yard, so I cleaned out their sandbox and got it ready to sell on Craigslist. I had my silkie chicks in a 40 gal long aquarium and got the idea that the sandbox might make an excellent brooder with a few easy "upgrades".
I used my husband's jigsaw but later realized that it would have been easier to just use a heavy duty shears as the saw was kind of overkill for cutting through the plastic. I cut out three of the large sections on each side to see my little cuties through, and to allow for airflow. Finally I cut out the big circle on top so it would have a spot to hang the heat lamp above.
Since I have a house cat, and I know they would eventually get big enough to jump out, I decided to cover the holes with hardware cloth. I cut pieces of hardware cloth that were about 1-2" larger than the holes for each spot. I used a drill to put holes every few inches along the edge of the openings and secured them snugly on the inside with small white zip ties, making sure there were no pointy edges sticking way out.
This type of brooder allowed my children, ages 2 and 4 to get up close and personal with the chicks - under my supervision of course. It was easy to just remove the lid and have all of us sit down for some great "playtime". As a result I feel these chicks are much more friendly than those I have kept in an aquarium brooder. I hope someone is able to use this idea or adapt it to their own sandbox!