I don't have much by way of experience with these plastic portajon deals, but for rafting trips i've used old ammo cans/rocket boxs that go by the name of "groovers" because the literally leave grooves in your backside. the tricky part is the lack of a release valve...and were therefore sometimes known to either slowly release on their own, or do so rather suddenly and violently, even when strapped down to the raft frame. anyway...
the real reason i'm posting is to share my shower idea. It's probably one of the cheaper options out there, and definitely good for the DIY'er. I took a couple of pieces of 2 and a half inch PVC tubing and the correct fittings to create a three rung "rocket launcher" lookin' thing that fit between the kayak stackers on my roof. the fittings were on the back end and all funnelled down to an on/off valve at the bottom that could screw into a hose. I stored the 4 foot section of hose in the bed of the truck and when i was using it, i'd either attach an on/off valve to the squirting end, so that i could control it from the ground, or just kink it when i didn't need water.
It held somewhere between 5 and 8 gallons, and seeing as it was painted black, it would heat up in the day, and after an evening of surfing, I'd have a nice warm rinse with a great sunset view. I didn't really worry about an enclosure...just a swimsuit.
Just a Thule sticker...kinda almost looks maybe professional