Andy,
Using the seach facility will bring up plenty of references to boat tents, of which the one highlighted by Matthew is probably the best. If you’ve seen photos of the MSR, then it sounds like you have already seen that thread.
You would think that someone, somewhere would be prepared to run up standard tents for BayRaiders. There are standard Drascombe tents but that maker, when asked a while back, couldn’t have been less interested. Living national treasure Roger Barnes (now in exile in France) made an interesting video of how he sewed his own boat tent out of ventile - see
https://www.youtube.com/watch?v=iDSEbhyIzX0. It was not without its difficulties and is not something that I would ever attempt.
My BR20 with standard sprayhood has been used in conjunction with an MSR Gear Shed (sorry, no further photos), an official Swallow tent and a C & J Marine boom awning. Of the three, the last has had by far the most use - it is the sturdiest, easiest to put up, take down and stow away, is pleasantly airy and has quite good headroom. It also doubles as a sunshade. If the weather’s really foul, then the fiddly Swallow tent does a good job of keeping the elements out. The MSR Gear Shed was a compromise until the official tent came along and is now only used pegged to solid ground together with its matching (and unusually named) Hubba Hubba tent.