There are even smaller and simpler CPUs that can be instantiated on an FPGA, which is as close to DIY-ing a CPU as plebs are likely to get. I own a "Precursor" that actually does this, have compiled it myself, but I want to go smaller and more hands-on. An 8-bit CPU on an Ice40 using an all-open-source toolchain the the dream, but might be a while...