We could possibly get the club to donate a shirt, then get it signed by as many Cas players (past and present) as possible.
That surely would be of significant enough value and a collectors item, imagine what someone would give for the signatures of Hardisty, Heppy, Hartley, Bryant, the Beardmore twins, Kevin Ward, Dean Sampson, Graham Steadman, John Joyner, Rangi Chase, Craig Huby's etc. etc. etc. signatures all on the same shirt?
Then put on ebay for an online auction?
...or if it's thought this wouldn't attract a significant enough amount, maybe an online raffle? given that every Cas fan would like to own such a shirt, £1 or £2 a ticket is not much to ask and could raise maybe upwards of £5,000 (or even more if we offered tickets to the rest of the rugby league world)
It's a pity it wasn't in the regular rugby season, as printed tickets (rather than online) could be sold at games.