I would love to do such a project in my free time, but I can't afford the needed software tools.
This started last summer and a number of interesting designs have been produced, pretty much entirely with free, open source tools.
Join the slack and ask around for projects that need help if you want to dip your toes in this sort of work! https://invite.skywater.tools/
I'm interested, but almost entirely on the analog side -- I'd love to design a NLTL, VCO, or distributed amplifier. I have the utmost respect for the crowd looking to scale RISC-V cores, but it's just not my scene.
EDIT: I took a look at https://github.com/yrrapt/caravel_amsat_txrx_ic_mpw2 and there are clearly some analog projects in there. Evidently I should have hustled harder!
That said, one could probably use the free (but not open source) Vivado HL WebPACK to create and test the RTL design and then follow along with the Skywater tools. But that way, the way towards the FPGA would still be closed source and only the path from FPGA on to ASIC would be open.
Further down the rabbit hole, I read about https://github.com/icebreaker-fpga/icebreaker which appears to be a tiny Teensy-compatible open source FPGA board with accompanying open source tools. Yay!
Thank you very much for bringing all of this to my attention :)
I also followed the Google/efabless program but it doesn't come with any handholding (kudos to the key google engineer who organized it .. he seemed very passionate from the talks/meetups I attended).
Ultimately, I got demotivated from my personal quest to fabricate something from scratch. I think FPGAs are the sweet spot .. they let you see what is going on without any of the physics.
Good to luck to you on your quest.
But the fab costs are still huge, especially if you don't get it right the first time.