Ok, this is not as fancy as a media center, but as a solution to the messy, saggy particleboard covered with peeling self-stick paper inside a bathroom sink cabinet it's quick and easy with a Kreg Jig. It took less than half an hour per cabinet, and I wasn't working very fast.
NOTE: If it's moldy and crumbling, you are going to have to replace the bottom shelf, not cover it up. (I'll post that when I get that bath fixed).
The problem with replacing the shelf under a sink is that the base was built into the cabinet and you can't easily insert a new one. Also, the center support in a cabinet prevents you from getting a single piece of material into position. The solution is 2 pieces, held snugly together with pocketholes and optional caulk.
1 – Measure the inside width and depth of the cabinet. If there is an open back, measure inside the wood framing, not to the wall.
2 – Select any combination of MDF shelf widths that will fill the cabinet from front to back.
My bath cabinet was 20", so I bought 10-inch shelving. You could also use a 12 and an 8, or whatever works
3 – Cut the shelf lengths to fit the space side to side. Square cuts, precisely measured!
4 – In the shelf that will be the FRONT piece, drill 3 or 4 pocket holes. One a couple inches from the end and one or two more spaced in the middle.
5 – Align the edges and clamp them.
6 – Insert the pocket screws.
7 – MARK the the rear board with something so you know which is the top side. I used masking tape with "TOP" written on it.
8 – Remove the pocket screws.You have just created alignment holes so you don't have to clamp the boards inside the cabinet.
9 – Clean the inside of the cabinet! You don't have to be hospital sterile, but you don't want dust and glooby stuff in there.
10 – Put the front board in, with the pocket holes on top.
11 – Place the rear board (the one without the pocketholes) into the rear of the sink cabinet, with the holes facing toward you.
12 – (optional) Run a thin line of bathroom caulk along the edge of the rear board.
13 – Align the ends of the boards and screw them together with the pocklet screws.
14 – If you use caulk, wipe off the excess.
15 – Put the white plastic cover plugs into the pocket holes.
16 – (optional) Attach 1/4 round molding to hide the slight gap between cabinet and boards.
Tah DAH! Clean and cleanable cabinet innards! Picture will follow as soon as I figure out how.
Oh heck - here's the link: