I used to have one, it was big and had a variable thermostat and had sand on the bottom. I gave it away before I moved never thought I would be back into propagation but I am and want a variable thermostat propagator but as small as possible.
I bought a couple of tomato plants last season but they were not at all as good as the ones I used to nurture. I have ordered a very tiny wooden `greenhouse` just big enough to bring seedling on to harden them off. In the old days, 70s. it was airing cupboard and in and out the house on a daily basis but having the propagator and greenhouses made it so much easier. I have downscaled bigtime but my genes are horticultural and will always be so. It will be used every year