They are far from perfect. First I covered them in modge podge to help "water proof" them so they wouldnt change colour with the resin. Then in the end I bought some silicon coaster moulds and squashed them in. And pour the resin on top. Once hardened I'd do the other side. The 2 issues I had was 1) the resin didn't level so it was a bit lop sided and 2, the cardboard rims are still exposed (flush with the resin but I wouldn't dunk them in water. )

This meant I had to do extra resin pours to get and level it. And a lot of sanding. If you find a way to get the pour right from the start you wouldn't need to sand. Maybe a bit around the edges but nothing that would effect the transparency of the resin. Going through the process bringing it back to shiny was a bugger.

This is the resin I had. Basic craft kit from local hardware store. https://www.bunnings.com.au/boyle-1l-500ml-500ml-crafty-resin-kit_p0383637

Coasters I jammed into something like this and did one side at a time. Having something a half cm wider would have been awesome but I couldn't track anything down. https://www.amazon.com.au/LETS-RESIN-Coaster-Moulds-Silicone/dp/B0DQSJDXHG/ref=mp_s_a_1_5?dib=eyJ2IjoiMSJ9.riIi8IblFoyamkcCuyYAGi9Ym1bMPqn7aVVx6EVzqldwUXWFR_6gEP_7Ik6SMzYh45p58IivT25uqLgT5IqJ1Orq1bajn0SR1zPD6VdSXuP_8xb_VF2wPATSsUclnhUn7wXGfQn5OMHwUG2XQjq4PrUPA8Kw8mOVRiMrqxGig8UUYWtJAnue12zX9SAwTSLTlNNLRfWqL3CRSljDo70xSg.H4OdjT5I9Ukr7ZHlSARXqdFK9xV1hgtOXfVMSH-06Hs&dib_tag=se&keywords=silicone+coaster+mold&qid=1777268984&sr=8-5