Hi Deb,
Somehow, you need to get a template for what you want. I’m thinking something with glass or a very smooth porcelain surface. Cut acrylic sheet or block could also work. Sometimes home improvement stores have this size and will cut for you.You could also look into having someone carve one out of wax for you (think jewelry making wax carvers). Make sure you specify you want a bright, shiny surface, which will require a certain wax and certain techniques.
Once you have your wax model, you can use it to make a silicone mold from. You can use the Alumilite High strength #2 or #3 to make your mold. You can find them in our store here: https://shop.resinobsession.com/collections/mold-making-materials
Otherwise, you could use a plastic mold that creates the shape you want, then glaze it with more resin or use resin gloss sealer spray to get it a shiny finish: https://shop.resinobsession.com/collections/tools-and-supplies/products/castin-craft-gloss-resin-sealer-spray
We have several shapes of paperweight molds here: https://shop.resinobsession.com/collections/molds/paperweight
Good luck!