The large capacity portapotties are available
in two models--portable, and permanently installed fitted for pumpout. The only other self-contained units that'll fit in your head are recirculating systems...and trust me--you DON'T want a recirculating toilet! So the only other viable option is a manual marine toilet and a remote holding tank. The bowl on any manual marine toilet that'll fit in your head will be about the same height as your small portapotty--whereas the larger portapotties are actually household height...and any tank big enough to hold at least as many flushes as 5-6 gallon portapotty will take up a considerable amount of storage space. There are definite advantages to a permanently installed pumpable portapotty on a small boat...they're vented, which makes odor control a LOT easier than in the sealed portable models...they hold about as many flushes as manual marine toilet connected to a 15-18 gallon tank...they're self contained, so they don't take up any storage space...no pump to maintain and periodically rebuild...and they require no plumbing except the vent and pumpout hose/deck fitting....so no sea water odor problems. All the advantages of a marine head and holding tank without any of the problems. And if it turns out to be too small, you can always add a second holding tank--much smaller than you'd need if it were the only tank--and reroute the pumpout hose to it with a pump inline (manual or electric).