First, they sell content, not reskins. This would cause an outrage in the spiral community, so-

Second, they still allow free to players to partake in this content, albeit at a reduced rate.

In Spiral Knights, new and intriguing weapons and dungeons are content. Ergo, you should be selling these.

I propose to you the following new format for prize boxes:

1.) Create a dungeon, using old monsters and mechanics, introducing new mechanics and monsters occasionally.
2.) Place prize boxes as rewards for these dungeons.

But how does this translate into cash?

3.) Each run of the limited time dungeon costs however much energy per person, like a shadow lair, but instead of a key, it's an entry price.
4.) From here, the sky is the limit. You can-
4a.) Sell an unlimited pass for x dollars
4b.) Sell smaller passes for x dollars
4c.) Sell passes on the energy market for however much

And lastly-

5.) Allow free 2 players to run the dungeon, and to get rewarded for running the dungeon one time only.

All of this accomplishes higher profits because-

A) Players actually EARN their prizes
B) Players aren't directly forced into purchases (unless they want whatever you're selling, of course)
C) It feels like there's content coming out
D) Players are not excluded from the "event"
E) It makes good use of old resources

Thanks for reading and I hope you give this serious consideration- things as they are now don't exactly breed happy feelings among many players, so it's only logical for you to consider other options.