Oh it only takes a very short time for them to make a mess of everything, get used to that' lol
You are limited to cross bars, but you might want to move it down one. Some times they will jump on and off fast and repeatedly.
While i imagine wooden trays could work, i think it might be a bad idea. They are likely going to poop and pee on it. Once that pee is in the wood, its not easy to get out. I learned that the hard way after trying to make a wooden hay container thing for my girls. They peed all over it and i had to through it out. All that work and it didn't even last a day. - Thats probably one reason so many use the metal Bass Pans.
You can arrange and style thing how ever you like of course, but ill post a pic to show you one of my cages, and why i did a few things.
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That is my boys cage. It has the same wheel and those metal Bass Pans. The pans are pretty nice. However i do have a sheet of fleece between the metal pan and the cage bars to reduce noise when they jump around. And there are wooden ledges around to make sure they can reach everything without a problem. And they are good to chew on too.
(Since the first pic didn't show the wheel too well.)
Technically that my girls wheel, but the cages are set up the same. You can see its positioning from the outside on the first one. Initially i had it just a hair lower, but when i got the bass pans the metal support beam would lay right on the pan lip. Metal on metal with jumping and running chins is a loud process. So now the top support bar for the wheel is just a hair above the cross bar to make sure the bottom support does not touch the pan lip.
The second one you can see the wooden support for the wheel. I wasn't quite sure the type of wood that was in the support they sent me, so i used some extra pieces of poplar i had and used some dowels to connect them all. The top piece has a little indent to hold the wheel bar securely. It helps support the wheel and reduce the noise from the bars on the cage walls.
Any way, that's what i did. Good luck with setting yours up!