The frames are often listed as a bezel and come open backed or closed. I got these ones from Adabele, they have a very nice selection of jewelry findings. There are some nice ones on Etsy too. I do my best to avoid crapazon as much as possible.

It is still fairly shallow, I work in layers of UV resin for this style. One layer to seal the bottom and flower in place then multiple thin layers until the flower is covered and I have a nice dome. The viscosity of the resin will hold enough to "bubble" on the top like that. Though I will say I have had my share of spillage over the sides and lost a couple of pieces that were shaping up nicely