A very great deal of the fun of this Summer's Wizarding Event was the so-called invisible creativity that provided much of the back story for the activities of the day. It also covered practical problems. In the case of the House Keys, it let us create a tale that explained why such a large and famous School of Magic was apparently contained in a 2-story building!
Somewhere in the early brainstorming for the event, someone put out the idea of the kids receiving a set of "House Keys". Unlike in Rowling's universe, which had students entering their respective Houses via a password and portrait door, our Houses were more prosaically entered using standard locks and keys.
Each of our 4 Houses had its colors: green & brown, white & yellow, red & orange, and blue & silver. These colors were incorporated in the banners and symbols used to designate each House. The keys, we thought, had to match.
Everyone has old keys for places they've lived or offices they've worked at or cars they've driven. I put out a call to the event cast and crew and other interested folks for KEYS! I ended up with over a hundred, which was most gratifying, as was the variety of their shapes and sizes.
Step 1 was to clean them. I filled a plastic container with hot soapy water to which I'd added just a tad of vinegar. Then, using a dish brush, I scrubbed all the keys, rinsed them with hot water, and set them out to dry on a towel.
Step 2 involved creating the matching sets of matching keys. As it turned out, there were enough key designs for me to create 5 sets per House. Each set contained three keys: two of them matching the two on each of the other sets for that same House and one mystery key. These last mystery keys were fun. Each student had a unique 3rd key. The story went that even the faculty weren't quite sure what doors they opened.
Step 3 involved spray painting the keys in their respective House colors. I worked out on our front porch using a large box to contain the paint mists as I worked. I gave two coats of paint to each set of five keys.
Once dried, the sets were hooked onto key rings (all but 5 of which were scrounged from the keys and key sets donated for the event). The third mystery key and a metal oval doo-dad my brother Woodcrafter donated were also hooked onto the rings.
The keys were ostensibly stored in an old, beat up box kept in the Headmaster's office. This box was actually a vintage wooden cigar box which I labeled using a typewriter font alphabet rubber stamp set and black pigment ink.
On the day of the event, the box was matter-of-factly brought out before the Sorting Ceremony. As each new wizarding student learned of their House designation, they were given their own personal set of keys to be kept ever after!