If you haven't fleshed and tanned the skin properly, it will only dry real stiff and disintegrate over time. You need to get that skin off the board immediately and either freeze it until you have time to work on it, or flesh and tan it up ASAP. I would suggest to freeze the skin for now (I hope it is not too late) and order yourself a tanning kit for reptiles. WASCO carries the true tan reptile tan which works great. Rittel's reptile tan does a great job too.
Another thing, timber rattlers are protected in several states as an endangered specie. You may want to make sure you have a legal specimen on hand. If it is not legal to possess, get rid of it fast.
