I used leftover Styrofoam from packaging. Glue or silicone the design you want onto a sheet of Styrofoam board, rough cut the edges you like with a razor, fine tune (melt) with a mini torch or work upside down above a large candle, cover with tile grout, paint and then seal.
Painting tips: Choose the right color grout, it will save you on paint. Wet brush dark colors and wipe off extra. Dry brush highlights. Use matte sealant to reduce shine.
Good luck, have fun and if you have questions let me know.
You can find many video's on youtube.com and this one is where I started out at https://www.youtube.com/watch?v=JUwZGfHMALM. Never got around to making them instead I just bought them. Good luck.