Why didn't I get that memo? Lol, I put the last piece of wood on the outside of my coop yesterday, will finish it up just in time for chicks in two weeksI like that you are using what you have available instead of spending scads of money. Reuse, repurpose, recycle. Your shelf unit will make nice nest boxes. You will want to close in the sides/back of the boxes to hold the bedding in. They should have a 4 inch lip across the front, as well as a perch of sorts so the birds can step from the perch into the nest box. That will prevent a lot of eggs. Each box should be a minimum of 12 x 12 x 12", but bigger is ok also. Some people even build community nests where a bunch of chickens can all climb in and have a hen party. Keep in mind that chickens seek out a fairly dark and secure looking place to lay their eggs. That will help you to close in the front of the boxes appropriately.
It would be helpful if you gave more details: Where do you live (general location for help with climate related advice). How much snow do you get in the winter? What is your average summer and winter climate/temp like? Do you have a dog who would patrol your yard for predators?
As PP stated, predators will be your biggest issue. A tarp will not stop any predator with teeth. A weasel will climb through any hole you could push a quarter through, and will kill an entire flock of birds in a single night just for the fun of it. Other predators are great diggers, so... a buried skirt around your kennel would be necessary.
You can find lots of free building materials simply by being a creative shopper: Call some carpenters. You may be able to get their building scraps, or remodeling debris that still has lots of life left in it. I can get lots of free windows every spring by visiting my town dump on a weekend. Other building materials are hit or miss there. My coop has lovely thermopane windows, and a full thermopane glass door, (metal, complete with dead bolt.) thanks to my town dump. Roofers often have 4' wide x varying lengths of ply wood left over from their roofing jobs. Habitat for humanity store often has lots of stuff that will help you to complete your build with minimal expense. Perhaps you can find a used dog house or small shed to provide predator proof coop, while using the kennel for a run.

Gary