This is based on a design that I first used in 2007. Certain seeds require heat to germinate and it is even sometimes possible to find research on line which gives the best germination temperature for individual species. The problem is that while heated propagators are available for sale, those that can actually regulate the temperature precisely are very expensive.
In this particular case I wanted to germinate the seeds of Citrus trifoliata also known as Poncirus trifoliata . It is a species of orange that is hardy in the UK where I live although the fruit are not wonderful.
My researches showed the best temperature for germination was 25 Celsius
This design overcomes this excessive cost by using a design involving an aquarium heater.
1. Plastic storage box.
2. Several bricks
3. An aquarium heater
4. A soil thermometer ( You would need one anyway with any propagator to ensure the right temperature.)
5. Seed tray ( One without drainage holes in the bottom)
It is a simple design, Your storage box needs to be big enough to that the aquarium heater can fit in it. Make sure you get an immersible heater or at least one that can be placed at an angle. I used a 200w heater. You may get away with less. There are various sources of information available for heaters in tropical fish tanks available on-line. These tell you how powerful a heater has to be for a given volume of water and a given temperature range.
I chose mine to be a bit more powerful as I knew the place it was being used was going to have a temperature of 10 Celsius or possibly a bit less at some times during the use of the propagator.
You put bricks in the bottom of the storage box put the seed tray on top. You then fill the box with water so that it goes up the side of the seed tray but doesn't flood it. Then you weight the seed tray down with another brick. This does lose a bit of space for the seeds but in this case it still worked well.
An aquarium heater is placed in the water and the top of the storage box is put in place.
You need to experiment a bit with the settings on the heater. I believe mine was quite accurate in its settings but some heat was lost in transmission to the soil. I had to set it at 28C to get the desired result (25C)on the thermometer which was placed in the soil in the seed tray.
You can see from the pictures that this worked well. I had good germination results. The picture with the young seedlings was taken after I had removed some to pot on.
Incidentally the seeds of this species seem to produce a very long root before you see a shoot and this is why some of the plants look rather straggly. They have been pushed out of the soil by the long root. This includes the seed!