I think it's pretty good as it is really. Bragging rights.. well.. you get a small icon.. something you could be proud of yourself if you know you actually won it in a fair way.
If there'd be some real price involved I'd definetly reccomend a better control over the time-limit.. But right now I think it's good as it is.
If it started at a certain time and people only had about 2h to complete their entry, then all the things such as rendering etc. would have to be included in the render.. also.. I think the numbers of participant would be pretty close to 0.. as very few would spot it in time.. and there's also those kind of people who have other things to do.
So I say: Leave it as it is, if someone gets a small icon from cheating... big deal
Cheers!