Marauder communication help
So, I'm writing a long series of letters written between Sirius and Remus. I'm writing one set during Christmas of their third year when Sirius ends up in the Hospital wing from eating too much Christmas pudding. I want Remus to send him a letter, but obviously he can't use an owl and this is before James, Sirius and Peter became animagi so Peter can turn into Wormtail and deliver it. So my question is, what other ways could Remus and Sirius exchange letters without owls and without being detected by Madame Pomfrey?