I've finished it! yay. This cart seat cover is indeed cute and I received a few admiring looks from other mamas during our shopping trip. I made a few mistakes in it but feel a bit proud :)
The pattern is A Universal Shopping Cart Cover from lil blue boo (free! thanks Ashley). It was a little pain to add patches of fabric to make it wider before diving into actual sewing but the end product is definitely worth it. I'd wanted use 60" wide fabric to minimize the preparation time but I couldn't find cute patterns. It doesn't show in the photo but the back side is simple dark brown fabric. If anything, I wish there were a slit or something where the handle is so I can have a firmer grip on the handle. Also because the cover pushes the belt down, it becomes too short, especially for my son who's tall for his age (Perhaps this would be a problem for him with or without the cover anyway). I might need to carry a separate belt for him.
But I LOVE this cover!
It's a bit embarrassing to admit my mistakes but these were what I did wrong.
1) I don't know what I was thinking but somehow I ended up with 4 leg openings (ugh) so I had to patch those up with leftover fabric (which happened to be where he sits so luckily it won't show when in use).
2) I'm still not good at attaching bias tapes. I'm not a fan of them. I didn't cut long enough bias tapes and had to connect a couple of them for one opening :( The stitching is not neat. Guess I need more practicing.
But I still like it. Actually, I like it maybe because of these mistakes. These mistakes show that I put my efforts in making something for my little guy. This is why handmade items are unique.
With extra padding, Jr. can now sit in the cart a little more comfortably. Moreover, since he doesn't touch the cart itself, he's not going to collect germs from the cart!