I don't know if it's the same as yours, but years ago I won a prize at a paddling club event: a humongous black boombox. Too big to fit in the hatch compartment, yup. Must've been designed for use on rafts or something with more cargo space than any kayak...except maybe a big double.
We used it on only one trip, when my husband strapped it on TOP of his rear deck. It has a vent spout, so beware that these things will STINK even in open air. If you decide you want to buy this type of boombox, I'd be happy to sell ours (it's been sanitized) for, oh, $10. I'm about 10 miles from Conifer via High Grade Rd.
Though later we made PVC tubes, what I actually use is Wag Bags. No reason you could not use Wag Bags with the PVC tube, to really cut down on stink. Better yet, buy the bulk Poo Powder and make your own baggie kits, since that would be cheaper, plus the Wag Bag bag is quite large. Someone I know says he can put 5 days' worth of poop in it.
I experimented at home with 2 days' worth. Let me tell ya that opening the bag on Day 2 is NOT something I care to repeat, so after I use up my stash of Wag Bags I will be ordering Poo Powder and making a homemade version with smaller bags!