Take a look at Strata Trust Company. Strata allows you to roll over your IRA into a custodian account with them. Once your account is established you can make investments via stock purchase or via SAFEs/Convertible Notes. The investment becomes "STRATA Trust Company, Custodian FBO Sam Malone Roth IRA."
I have personally used Strata to make investments in my company as well as others. Strata and other Custodian companies currently hold close to $1M in investments with us.
Strata is not the only company out there. I know of Forge, and CamaPlan, but Strata has been the best to work with.
I think they charge an annual fee of $500to manage the account for you.
I'm attaching a form Strata uses for investments so you get an idea of what's required.
Its been super easy to do - if you have questions, feel free to reach out to me and I'll gladly discuss more in detail.