There are portable portapotties...and then there
are portapotties that can be permanently installed and fitted for pumpout...you wouldn't be able to bypass the tank to flush directly overboard, but you can install a y-valve and macerator or manual pump in the pumpout line that will allow you to dump it at sea. I can't blame you for not wanting to mess with a portable tank...but IMO, a self-contained model fitted for pumpout makes the most sense on the boat that small...'cuz a toilet, all the plumbing and a tank would eat up close to half the storage space on your boat. A self-contained system doesn't require a flush water intake thru-hull...no plumbing except the pumpout/dumpout line and vent, no toilet maintenance...a 5-6 gal model holds about as many flushes as a manual marine toilet would put into a 12-15 gal tank (anything smaller wouldn't be worth the effort and expense)...and would cost you only about $250--if that much) for the toilet/tank, hoses, pumout fitting, and any other parts. A decent quality manual toilet and good quality tank, intake thru-hull and seacock, hoses, and all the other parts will run you at least twice that much.With a self-contained system, you have all the same advantages (ok, you wouldn't be able to flush directly overboard at sea, but you can dump the tank easily enough) of "big boat" system with none of the maintenance and problems...AND you don't lose any storage space.The SeaLand 965MSD (be sure it IS the "MSD" model...that means it can be permanently installed and fitted for pumpout...the 965 without the MSD designation cannot be) is the best quality. The fittings are even designed to allow you to disconnect 'em and take the tank ashore in a crunch.