Here's a simple solution that I came up with to take care of that same problem.
Cut a board to go from front to back or side to side of each tray. Cut the board about 1/16 - 1/8 inch longer than needed so it can be wedged in place.
Cut a piece of plywood or heavy board the length & width needed to cover the feed area, fill up the area with feed & place the board over it. (See photos below)
When I built my rack I ripped 2 x 4s in half so each tray's frame extends 1 1/4 inch above the screen. After putting a decent layer of feed in my feed area the 3/4 inch OSB board I use as a cover still sits within that frame, making it harder for wild/stray rats to lift it up & get to the feed. You can always weight the cover board down with somehting.
With a 1:3 ratio of rats, and their babies, I only have to refill my trays about once every 5 days or so.


Yell if you've got any questions.
Take care!
HH
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Due to political correctness run amuck,
this ol' hillbilly is now referred to as an:
Appalachian American

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