Hi Giles
I invested in 2 Super 7 heated propagator's last year, they are long and thin and each has 7 little trays for sowing a variety of seed at the same time, and when placed side by side on my south facing kitchen windowsill they fit perfectly (much to the annoyance of my Mrs) but if you don't want to go to the expence? I used to just place 2 half seed trays inside a full seed tray, sow my seed and cover with either cling film or place inside a plastic bag which did the job just as well!! It's just having the advantage of a bit of bottom heat, it means I can start sowing a little earlier than I used to be able to.
Hope this helps!!
Happy Gardening!!