If you are looking for a solid bottom why not cut a piece of plywood to fit? Personally I would cut the entire bottom out and let them eat the grass.

Plywood wouldn't work.its round why not go to a home center maybe a piece of vinyl,tin etc I also would not put new chicks on ground .The chicks are fragile for a few weeks.We made a brooder a big wooden box on legs with a screen top .Youre gonna have to have a light in there the depth may be too low in the feeder trough.You need to read up before you get them.

oh yeah been reading and STILL reading. I usually don't jump into anything until I find out details and how to's. Yeah I knew not to put them on the ground with them being little, the ants around here would carry them off! In fact they got me today and I NEED to do SOMETHING about them to get them gone but have no clue as to what since I don't want nothing poison on the ground. Got 1 dog and 3 cats and going to get the bitties to.
Might cut the rest of the bottom out of this trough to use later on for the bitties when they get older. I can put some 10% sevin dust around it to keep the ants out I think when they get older.
yep knew about the light. :-)

Well my sweet nephew brought us two shipping crates from where he works. The 2nd one he brought was deeper and larger so I just took it apart since it was screwed together anyway and we made it the same size as the other one was. THEN we cut out a window on both brooder boxes so we could put in a mesh wire screen so the two groups could see one another and it'd be like they were growing up together. They were 2 weeks apart so I couldn't put them together. We screwed the two brooder boxes together so it worked out fairly well.

Anyway now they are all out in the BIG chicken pen. They all have out grown their brooder box. Growing like weeds now and loving all the space in the big chicken pen. ;-)