Help on a arts & crafts project please?

I am making a Toy Chest for my granddaughter and have routered a heart shape on the front, this is only 1/4 inch deep and I was going to paint it red. I decided to try and infill it so that it looks solid. My question is : does anybody know a liquid i can pour in and it sets hard overnight say, I have tried mixing red paint with fller but it wont go hard. I would be grateful if anyone knows of a product that appears glossy once it has set, if I cant dye it red then I will paint it after. Any solutions are very welcome.