I made a 3-piece platform for my basement setup, and haven't had any issues with it in the last 6 months or so.

There are some great videos online from those who have made better ones, but my design was straightforward. The middle section matched the footprint of the inside of my rack - so start with those measurements. This is two layers of 3/4 inch plywood. If you're concerned about cosmetics, use some hardwood (or veneered) for the top layer of it. This movable section allows me to simply slide the platform into the rack for squats. (I needed a bit more height due to my j-hook spacing, and I like standing on wood vs. concrete.) The two outer sections should be about 2 feet wide, but that will depend on the middle section's dimensions as well as the size of the sheet of plywood you'll start with. From there, I topped the two side sections of 3/4 inch plywood with 3/4 inch rubber mats of the same size. They can be trimmed with a utility knife or a jigsaw. For both the wood/wood and wood/rubber, use some commercial grade adhesive and the appropriately-sized finishing screws/washers. Stack some weights on them to help the adhesive set, and have at it.

Hope this helps. YouTube is definitely your friend if you need a more detailed step-by-step or other ideas.