We've lived in this house for about 7 years. For 6 1/2 of those, we had a trash compactor taking up space in the kitchen. We never, ever used it. I kept meaning to take it out and replace it with shelves and it took that long to finally get around to it.
Removing it was easy. Taking a sample cabinet door to a wood store was easy. Buying the 3/4" cherry plywood was expensive, $80 for a 4' x 8' sheet, but easy. Cutting the plywood was hard.
First, I don't own a table saw. I wish I did, but I don't. I had to cut the sheet into shelves with a circular saw. Meh. I bought a device that was supposed to hold it straight at a specific distance from an edge, but it didn't work well at all. I ended up trying to cut long, straight lines by hand.
Second, the opening isn't square in any way, shape or form. The front is narrower than the back and the bottom is narrower than the top. Whoever did our cabinets, they weren't top-notch craftsmen.
Luckily, I had more than enough cherry plywood, so I was able to cut until I got pieces that fit. Installing them was easy. Staining them was a dirty job, but likewise easy.
Putting a polyurethane coating on them was a bit more difficult and I did a poor job of it. There were tiny bubbles in the finish, but by this stage in the process, I was way behind schedule and I just wanted to get the shelves in and go on to something else. Instead of sanding out the bubbles and doing another coat, I tossed them in and put pots and pans on them. You can tell I'm an amateur if you run your hands over their surfaces, but otherwise, I think they look pretty good.