Perfect Drift great questions!
I've built a few small frames like you discribed for a set of Wing 10'(the best tubes around) they are not a huge diameter something around 16-17" ,But these tubes were designed for white water,so depending on the frame componets you want,i.e anchor system,some way to stand up (not what you think),etc etc. I assume you'll be around 40-50lbs maybe less.These smaller frames have their place just like you've discribed.I won't go into design features here,thats what make my frames so unique;)
The boat has to fit you're needs,good luck....Os

btw as far as an over the counter boat for what do what I'd look at the Steelheader,
IMHO he has a few design flaws.....
That's why I built my own:D
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[/b]The less I give a [Bleeeeep!] the happier I am[/b]