@Darryl S.
I took it from the comment "I'm sick of paying these short term capital gains" as an indication that you were - in addition to your full time employment - flipping houses personally. That activity would be producing the self employment income (it's not cap gains) that is necessary to sponsor a Solo 401k plan. By deferring some of that flip income into a Solo 401k, you would reduce your tax profile there.
I don't see how UBTI "ruins your Roth IRA plan". If someone said,"I can show you how to reliably generate 40% ROI tax free to your IRA in a few months time", most people would be jumping up and down and shouting for joy.
Sure, it would be really, really nice to be able to keep all of the initial 60% ROI this particular deal could produce, but that just is not the way it works. If we let tax-exempt entities compete with taxpaying businesses on an uneven playing field, we'd have no tax paying businesses at all left and then imagine what your personal tax rate would look like?
If your Roth IRA is making 40% today, please let me know how you are doing that? If you would rather your Roth IRA have that kind of potential, contact and consult with one of the self directed advisors available to you through BP.
We very frequently speak with investors who have the potential to do really, really good things for their IRA just by using it to do something they are already having success with. Sadly, many throw that opportunity away and leave their IRA in some under-performing portfolio of paper assets simply because they see or hear one thing they do not like about the IRS restrictions that come with these tax-sheltered savings vehicles - such as UBTI or the prohibitions against sweat equity. IRA investing is different. But is can be really rewarding too.