I feel like the shorter entry period and the less frequent rounds are kind of at cross purposes. From my own experience, a MotF entry doesn't usually take more than 4-5 days of actual work - it's a matter of having a suitable idea and finding time for that 4-5 days of effort within the two-week timeframe.
For some reference, a few years ago when I had more time on my hands, I tried running a
weekly map contest. While some rounds we got a bunch of awesome maps, some rounds there were just a single entry - maybe the theme didn't appeal, or maybe no one had time that week. If you're doing a new round every week, you can sort of roll with that - maybe the next theme will spark more interest. But if the next round is four months away, then a contest where maybe one person submits an entry is going to be kind of a flop.