> If I buy a good loose lab grown diamond will I be able to find someone who will fit it into a high quality gold ring?

Yes, and even better, don't design the ring by yourself. Get a nice jewelry box for the diamond, use it for the proposal, and when you open the box, say:

"Our relationship is something we're going to build together. I want your opinion on everything for the rest of my life, because you're going to be my partner. I got the diamond, but let's design the ring together, you and I, because this is too important for me to do by myself. I need your help."

They'll melt, and the ring will mean even more.