I know this has already been answered before multiple times, but I think I might have found a solution. Granted it would probably be about 2 inches thick if not more.. but the desktop replacement laptops that are out already are comparable to that. Everything I have looked up to be low profile, has been under 2 inches, that includes ram, cpu cooler, gpu, and even the I/O shield. That only leaves power. I found a 500W pico psu. Is there a way to connect two laptop batteries together or something... and connect that to the psu as well.
http/www.aliexpress.com/store/product/Free-Shipping-X7-ATX-400-400W-High-Power-24PIN-DC-ATX-Power-Supply/211500_747813627.html
http/www.aliexpress.com/store/product/Free-Shipping-X7-ATX-400-400W-High-Power-24PIN-DC-ATX-Power-Supply/211500_747813627.html