How about using aquarium thermostats? They could be used several ways, for instance, instead of placing them directly into the trays, use an over-size tray to set a container of water in one end, with the thermostat heating the water which in turn heats the tray. This has the advantage that you can switch between trays as needed with a single unit, without contaminating your chemistry.
You could also make a sort of floating lid out of sheet foam insulation, with tray-sized openings. Bricks could also be employed as supporting spacers as well. Partially fill your sink, thermostat installed and let a water-jacket keep things warm. You will want to insulate your sink but it would be worth the effort and there are a number of easy to use products out there to do the job with.
You might also consider a large ice-chest for keeping chemistry from cooling off so much that it takes forever for it to come to working temperature. If you use a aquarium thermostat here, again in a jug of water, you should be able to keep things warm as well, though you'd need to compromise the cooler a bit for running the cord through a hole or lid that doesn't quite close tight.
Another option might be to build a 4-5 inch thick, closed section or box directly under your sink,with a light-baffled box lower down and off to one side, where you could keep a 60 watt incandescent bulb or better on to warm the underside of the sink. By having it lower down and well insulated, convection heating of your sinks' underside should be possible. You would most likely need to include a sliding vent cover at the far end of the warming box to assist convection and regulate the heating. A dimmer switch will help too. Just be careful to build the lamp enclosure to avoid an overheating fire hazard.
Eli
If I remember from chemistry practicals, simple chemical reactions by and large proceed at a speed governed by temperature, so increasing your development time by a factor per degree below 20 degrees should bring you to the right degree of development. Since installing a new efficient boiler my cellar darkroom temperature has dropped from around 20 degrees to 16 degrees and I have arbitritarily increased dev and fix times by 25 per cent. Seems to work. Any chemists out there can help?
Richard
I use a torpedo heater for the garage & it uses kerosene fuel, I think that's what you folk call paraffin.
Kerosene heaters come in a variety of sizes. the torpedo has a fan that moves a lot of air, but there are units that are about 18-24" tall X 10-12" diameter without a fan that work very well. Usually they have handles to move them around with & are not heavy. There may be a visible orange glow with them though. It is orange & MAY be safe around paper. Definitely not film.
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