The closer you get your wood to 0% moisture the better it will work. After all the resin can only go where water would have been. The problem with that is of course checking. Riser blocks can be a pain to get dry enough to warrant doing it.
The cracks you get in the 'lid' are not a problem if you use the correct material. In fact they are normal. A lot of the commercially available setup will appear 'cracked' after a few hours at vacuum.