My Nova has three chemical slots plus a wash slot, which is handy.
AS for the cling film, maybe I'll have to try again. Do you still use the lids or just cling film? I think I used clingfilm over the floating lids and that did cause a bit of mess.
One trick to help Clingfilm to work is to radius the sharp corners at the top so they don't pierce the film this also helps to get a good seal. My method of applying the film is to ensure the ends of the tank are perfectly clean and dry as well as the lips of the outer chemical baths, plus the tops between the different slots. Now taking the roll of film apply about 3-4 inches at the rear of the tank and roll the film towards you and bring the roll over the front by about 3-4 inches. Cut it cleanly with a craft knife or similar.
Now taking the outer edge of the film at the bleach side in one hand, press it into close contact with the outer edge and the top of the right hand slot. Stretch it tight (a must) and bring it into contact with the next strip and finally the edge of the dev slot. Try to get it smooth (not easy I know), but once that is done you should be able to have a relatively air tight seal. Yes, I leave the floating lids in place as well, the less exposure to air the better for at least the dev. As the tank cools down if the seal is good you should be able to see the film curve inward to the slots caused by a near vacuum as the air cools.
It also helps if the chemicals DO NOT come above the middle line of the lids.
When the next time you go to use the processor, remove the film first before switching on because as the processor heats up, some chemical will evaporate onto the film and that will cause a mess when it is removed. With good careful replenishment, the life of the developer etc will be extended into several month at least, if not years. The only thing is, it does have to be drained, cleared out and cleaned once in a while. I do mine about every 6 months. I just refil the slots with the original chemicals and top up if required. You can never over replenish.
The comment made previously about the two slot processors being mainly unheated is not strictly correct. When the NOVA slot processors were first conceived way back perhaps 35 years ago, they were intended for RA4 processing only. B&W was still largely done in open trays or automated machines in commercial premises so there was no need for another system. The unheated processors only came into production a good while later.