OP here,
Not that anyone cares, but I decided to build a deadlift platform this past weekend. Very pleased with how it turned out! I used two layers of 5/8" OSB on the bottom, one 3/4" layer of birch ply for the top 4x4 piece, and cut two 2x4 horse strips of 3/4" horse stall matting for the end pieces. It's all screwed together and extremely solid.
A few of my first impressions:
1. I can see how people have issues with the different layers warping/not laying perfectly flat. I took a lot of time to make sure I got really flat pieces; however, who knows how they'll hold up over time. To be continued, but it definitely does the job. I decided to keep the platform on top of my already covered floor (stall mats), so the concrete will never be in danger now (two layers of 5/8 OSB + two stall mats).
2. Maybe my favorite part about it is just having a dedicated space to deadlift. The thing looks awesome too. I lacquered the top piece of plywood to protect it, and it turned out a lot nicer than I thought it would. I am keeping my full deadlift jack on it with a bar, ready to go. Load it up and lift. I thought about keeping a pair of 45's loaded and ready to lift off of the ground. What I consider convenient and motivating, you may call a waste of space.
3. The cost wasn't much, really. I spent $17.50 per piece of OSB ($35.00) + one sheet of nice plywood ($40.00; I only used half of the sheet, so....$20?) + one horse mat on sale for $35.00. So, between $90-110 depending on your view of using half of the birch plywood. I can use it for another project.