I've come up with a design that uses grooves, so I think I'll be going with that. A couple more questions...
What are the best woods to use for the frame? I'm no wood expert, and I keep thinking of ash, but the local timber place has white oak for quite a bit less (they have it in stock, whereas the ash has to be cut and dressed from a larger piece). It seems to me that the main forces to consider are flexing of the uprights due to the tensions in the blade and cord, and lengthwise compression of the crossbar. I don't know how strong these forces are, other than a vague recollection of the tension in a bow saw I used in the scouts many years ago. Would oak be fine for this?
How solid and structurally stable are typical buck saws? I've never used one, so don't know what they feel like. Is there likely to be any wobble, such that they have to be handled with a bit of care? Or can a fairly basic (ie, imprecise) assembly be used to beast into decent sized logs without worrying about things like the blade going twisty, the bits parallelogramming, etc? I've seen some that just have sqaure notches in the uprights, others with round pegs and round holes, and some that use mortice and tenon. I'd have thought the latter would be best, but is there much difference in practice?