How to make test tube macaroni?

I mean those they sell at fairs. It is a tes tube filled with colored gel and have macaroni letter inserted into them one by one by using a skewer. I have seen people done it but a friend told me her attempt at it was a total flop. The macaroni rotted. I would like to make my own (DIY! Yeah!) but can someone tell me what type of gel should I use and whether the macaroni ought to be treated to prevent them rotting. Thank you in advance!