They're really pretty ! Your moulds are better than the Amazon ones, because they give more support, while the Amazon ones will bulge out. You could try putting them in a tray and packing around them with sand to stop them bulging.

I'm sure I saw a video on YouTube ages ago from a chap who fitted drinking straws to the filler holes of this type of mould. It created a small "head" of pressure which helped reduce the number of air bubbles slightly, and also created a sprue which could be clipped off. That made more work sanding down the nubs, but made for a cleaner end result and reduced the number of dice with imperfections around the point where the resin was poured in.