I have been using toilet paper rolls for my first stage nursery for years. Last year I moved, and since I had no idea where I would land I recycled them.
Now I have only a few, nowhere near enough for spring planting.
After a great deal of thought, I have purchased a silicon ice mold and will be using it to make soil blocks. I will be using the native soil (heavy clay loam) and would like to hear from people who have used soil blocks in the past.
I’ve used soil blocks, but I used a mixture of peat, perlite, and compost, with some native soil (clay) added to help them hold together. My guess is that clay soil by itself would compact too much to work for seed starting blocks.
With the peat-based mix, the blocks worked really well for me.
I use soil blocks, and try to not buy anything. It was hard to get away from at least a little coco though (but sometimes that was just a lack-of-time problem). The thing that made it work for me if I had the time, was sifted oak leaf mold. I had access to a tan oak forest where I would get bins of rotting leaves +soil. Then sift that and make it a heavy part of the mix. Instead of perlite I used composted biochar for more aeration. That worked, but again I was lucky with that. I did some blocks of tomatoes with pure soil, they were really stunted because of lack of oxygen in the root zones. So I think you’ll do fine if you can just find some source of aeration for your clay.
Here’s a random video. Sand is in the recipe here, from Elliot Colemans recipe, but I stopped needing it.