They come in various sizes and I re-use the trays from year to year and just buy the boxed pellets.
My first year was a moderate success.
My second year wasn't very good for a few reasons. Long, cold, wet spring, and cool summer. Add to this the fact the previous year high-nitrogen demanding watermelons depleted all the nitrogen in the soil.
This year I had great success with indoor plantings using off the shelf potting soil. The seedlings were spectacular. But when I put them in the ground they were stunted.
I suspect the fresh potting soil had the nitrogen needed for the watermelon to grow but the outside soil didn't.
I'm going to try again next year but I will be applying a couple different high-nitrogen fertilizers in the spring while there's still snow and let it slowly disperse as the snow melts.
I strongly suspect watermelon require far more nitrogen than other plants.
Armed with my research I'm going to give it another shot.
Maybe you can give it one more shot using high levels of nitrogen and see what happens.
Do a small planting with extra nitrogen. Ask your friends to try.
They come in various sizes and I re-use the trays from year to year and just buy the boxed pellets.