Spread some butter, or jam, or jelly, on the sponge to stop the liquid soaking through before it sets.
Agar agar works like gelatine - it's the vegetarian version But I would go with arrowroot.
If I don't know what a word is in another language, I use Wiki - but there is no entry in French for arrowroot. However, it does give the Latin name Maranta arundinacea, which would tally with the "marante" you found, Tricia.