If you cut a slot in the tube, you could potentially slip it right over the Hobie Padeye (or a stainless eyebolt) and use a hingepin or dowel to keep things together.
Or do the same thing with a flat alum bar, that could be screwed neatly to the seat and bent to conform to the cockpit shape.
I looked at those two options as well. With v0.2 of the design I cut the original cleats (in the spot that I now have the pipe-clips attached) down so they were just small pegs sticking up out of the gunwales, and drilled a matching hole in the ends of the crossbar, which them slotted over the 'pegs'. It was held in place by the velcro/clips, but I found that the crossbar would sometimes twist and ride up the 'pegs' slightly.
I tried some 40x5mm ally bar, but it seemed to flex too much. Box-section 45x20mm was another option, but it would've required a bit of cutting and welding, to make a nice fit - which is way beyond me! The only way to lower it would be to shift the entire seat forward by about 50mm, but then it's staring to get in the way of the steering and seating.
Although the seating position looks quite high, it's actually pretty much perfect for me. If I lowered the front of the seat, I'd be sitting to upright (particularly when wearing my PFD). One great advantage of a high seating position, that I'd never really thought about, is how easy it is to stand up. Rather than having to clamber up from the almost recumbent 'original' seat, it's now a simple case of simply shifting forward and standing - in the same way you'd stand up from a lounge, or dining chair. Same goes for sitting down, just SIT DOWN - no farting around stumbling into the old Hobie seat.
Of course, it's not all plain sailing - when seated, the lower edge of the sail is at shoulder height, which can be a bit annoying. I'm toying with the idea of extending the mast by about 30cm, so the sail is raised above head height, but I haven't investigated if this will cause other issues.
The advantage of the current version (having the crossbars simply slot into the pipe clips) is that it doesn't matter if the crossbar twists as it's mounted in the 'circular grip' of the clips. In fact it makes a handy pivot-point, as you can flip up the entire seat, from the back, and stow stuff under it, without having to force it under the seat front - if you know what I mean?
Ideally, a full clamp, like the ones used for our new Aka cross tubes or the smaller Hobie "sidekick" tubes would make the seat fully secure.
I have a couple of similar clamps, but they were too long to fit comfortably in the recess for the padeyes - I guess I could pad it out from underneath. I could make quick-release mechanisms for these clamps, with wing-nuts, but it's starting to add to the complexity a bit.
Does this get removed after each sail?
Yep, and it only takes a few seconds to remove (or fit), which is one of the best features.
Thanks muchly for your comments and feedback!