I bought a Si-Loc belt on the recommendation of a pelvic PT. It’s designed for this purpose. She said it works for some folks, not for others. Personally I’ve found that, after a few weeks of regular use, it has relieved most of my SPD pain.

I opted for the maternity version: https://www.amazon.com/dp/B005NZ0PUM/ref=emc_b_5_mob_t