I do have some small naturalised clumps of nivalis in the garden, but all of my “collection” snowdrops are grown in large, round aquatic baskets around the perimeter of brick built raised beds. This affords me the benefit of being able to lift and divide my snowdrops when dormant.
When you plant a basket, dig out a hole that is slightly deeper and wider than the intended basket, put at least one inch of sand in the bottom of the hole and infill around the sides of the basket with the same compost mix that you are going to put in the basket. Make sure that the basket is not lower than the surrounding soil.
The size of the basket will be a personal choice, but be prepared to lift and divide frequently.