I'd imagine that they do have several closures, but I can assure you that you can order right up until the last minute (like I did, this time last year!).
The order process can take a while if you want a bunch of options. I'd suggest:
Get it together what you want, and put it in writing. You might try printing the results from the custom quote generator.
If you are going to work with a dealer, call them to discuss your order, and Alembic will call you if they need clarification. Alternately, if you want to buy direct, you can just call the mother ship.
If you are buying from a dealer, you should call the dealer first. There are several that really know their stuff; I would not hesitate to recommend calling Beaver at Bass Central, or BassNW (Evan or Chad).
I wouldn't suggest hesitating, since last year it was pretty hectic.
Brad
