Unlike BMbikerider my experience with both Nova's own developer and Ilford's MG developer is that the developer lasts a lot longer than a day. The tube that you put on top of the slots keeps the air away very effectively and if you are not developing for several days then a quick squirt of Protectan or other air-removing gas and then cling film will be effective.
Just make sure that on the next printing session you replenish the developer according to the maker's instructions i.e. if its 10mls per 10x8 print and you have done say 5 prints at your last session then fill up with 50mls of fresh developer.
I have had my Nova for about 9 years now and have never regretted its purchase