Pretty simple. Basically, give Brinks (token trader) a new type of token that can be exchanged - a surprise token. The only item in the surprise token area is a "surprise box" - potentially from various years (2014, 2015, 2016, etc). This is exactly what they have on the tin - the surprise boxes that show up once a year, typically for Spiral Knights' anniversary.

There are a few ways to obtain these tokens. First, through the supply depot, by purchasing one token for 1500 energy. A second option would be by allowing forge sparks, grim sparks, and primal sparks to be traded in for surprise tokens at something like 150 sparks per surprise token. Third option would be via featured auctions, putting a single token up at 75k crowns bid every so often, or a batch of 5 of them for 130k crowns buyout each.

Boxes themselves would cost 3-5 surprise tokens each from Brinks.

This would serve as a way for older promos to be able to see the light of day again, and allow F2P players access to the joys that are promotional boxes, however they would require a heavy time investment, a watchful eye on the auction house, or active grinding for energy.

Numbers can be tweaked around so they're balanced, but I feel like they money costs should be balanced around the spark token grind more than anything.