English perennials expert Alan Bloom used buried plastic liners to grow plants wanting damp ground, called it I believe "Moisture Gardening" in his book on the subject. I suggest you try something like that if growing the primroses is the main objective, rather than trying to establish and maintain a tiny area of open water.

Unless you want that anyway. If you do put in a pool keeping it topped off may not be as much of a hassle as all the mosquitoes that will try to breed in it.

If you install fish to control mosquitoes then you may have herons coming to eat the fish, probably all the more so if they are brightly colored goldfish or carp. Raccoons tend to be interested in pools and ponds also:

If You Build It They Will Come

If there is a water gardening specialist with a retail shop within reasonable driving distance maybe go see what they have to say. One near me ended up putting wire over the main outdoor koi tanks but apparently wasn't having much problem with unwanted visitors to small decorative pools right by the entrance and shop as I don't remember those being covered.

With that small an area, the pond depth would dominate the water replenish rate. Perhaps tweaking the rate of a small drip irrigation device would keep the system in balance, or use some kind of water level sensor to turn the water source on and off.

A friend of mine used a water level sensor from a toilet to keep his small pond at a certain level. I can see where it would work to feed a bog. He also used water from a downspout, but not in the winter. He didn't need it then.

Mike