Lazarette location is ok
You don't want the tank TOO high--an uphill run from the toilet leaves waste in the toilet discharge line that can seep past the joker valve back into the toilet. If the head is forward in the cabin, the best location is under the v-berth...if it's aft, high in a cockpit lazarette is a good location 'cuz it allows the vent line to be short and very close to horizontal (for a full explanation of why that's a good idea, read the article "Odor Control in Holding Tanks--Odor Out the Vent" in the Head Mistress forum Reference Library (on the forum home page)...you'll also find the reasons why burying it under the cabin sole is a bad idea--ok for fuel or water, but not for a holding tank. However, it's VERY important that the tank not be more than about 6-8' from the toilet....that's about as far as anyone will ever keep pumping. What you really want is a sturdy thick-walled (at least 3/8") polyethylene tank, not fiberglass. Ronco Plastics is your best source...they make top quality tanks for a very reasonable price, and have more than 400 shapes and sizes to choose from--more than 100 of which are non-rectangular. About half their catalog is on their website at http://www.ronco-plastics.com If you can't find one that will fit there, call 'em for a full catalog.