FWIW, the US Military was sold on the bed liner to prevent spalling and used to require it on almost all vehicular armor, after they actually tested it, full reversal, it's banned from use in most contracts. It spalls every bit as badly, and in many cases worse than the original surface.
Real deal SAPI plates are expensive, but they flat out work. Lightweight, and easy to wear, 22 pounds of steel is a lot of weight to hump around, especially when you're probably still going to be badly injured by the spall anyway. If you have an honest need for body armor, you need body armor that works.
Be wary of any low cost SAPI plates, there are tons of them failing and being rejected by the DoD, I wouldn't doubt that they're being sold on the civilian market.