Fish jumping into the filter compartment in biocube

[edikpok]

I have a 29g biocube and today was the 3rd time that a fish jumped into the filter compartment. Although I saved it (can't say the same about the poor clown that tried the same trick 2 months ago), I want to purchase or make a little guard for fish to not jump there. Where can I buy such a plastic guard ? if it is a DIY project, what plastic should I use?

Thanks!