Replaced the stock 15mm x 15mm x 10mm heatsink that came with my Pi 3 B+ kit. The 25mm hs fits comfortably on the Pi 3B+ and has enough room to fit an additional 25mm above the USB/network controller (with a little movement). A few hours later the simple software that runs it reported that it was 3 degrees lower with the same summer weather ). . Mission accomplished.