My new cat project is in the works! (A cat-run for Pierre!)

I remember reading that I needed to post two new topics in this forum before venturing out, so...here goes!

Recently, Pierre (my feral cat) has been exhibiting signs of sadness. I have taken him outside several times in the past two weeks and each time we go outside, he eats tons of grass, growls when it's time to go inside & then whines by the sliding door when he isn't outside. He spends A LOT of time by the windows and when anyone goes outside to the back, he'll chase them from behind and hope to catch the door opening to get out. He won't move from the sliding door until they get back in & meows sadly until they do. Pierre has never done this, but I'm putting all my eggs in one basket and assuming that he is just DYING to get outdoors. (He's my first cat and is strictly indoor)

If Pierre was a tame and mild mannered kitty, I'd have no problems letting him go out - but I know he won't come back. And that's just too much for me to handle.

Where I'm going with this long winded story is.....I'm going to build Pierre a CAT RUN! I know that this would solve all of his little emotional outbursts & it'd make me feel loads better knowing he'd be happy. I have seen dozens of professionally built cat-runs and DIY ones as well and I know I can make one! I would like to know if any of ya'll have one, if you've made one and any recommendations you may be able to give me. I am very excited to begin this project for my little friend.