Open-source chips is probably the difficult part about it. There's RISCV CPUs with open-source high-level description - but the physical devices are not open-source (for example the physical layout). There's some people running soft-CPUs on FPGAs. This way the circuit of the CPU is open-source but the FPGA fabric is not. I believe (and/or hope) that the situation might improve in the next decades. There's some chip fabs which start to allow creating fully open-source chips (Skywater in the US, IHP in EU). Yes, most chip fabs are actually forbidding to create open-source chips.

This might be inspiring: https://mntre.com/

Theoretically current RISCV boards are not fully open since the processor itself is not. That might or might not matter to you, but you can always use an FPGA board. Anyway, hmu for anything, it's nice to help with projects as a community :)