Are you handy with the construction of said box?
Should not be to hard to make.
You would need some 2x4's, and a couple sheets of 3/4 plywood.
Make the back higher than the front for the slant. If you make it slanted enough, the chickens will not be able to perch on top of it.
Make it tight so no rodents can take up residence.
You could use nails, 2 inch staples (if you have the air gun) or wood screws. The screws mKe it stronger IMO.
just measure the spot you want it to set in. Draw it out on paper and go for it.
Good luck!
PS. I use the 5 gallon buckes for my feed. Easier for me to handle and keeps mice out. Also the buckets double as a seat for me to sit on and comune with the girls at feeding time.