Assuming you are using float bubbles when in 'spin mode' a 2 or 3wt may be too light for casting distances (adding water to the bubble trying for more distance on a mountain lake for example).
If you want something packable the Batson RX6 5 piece fly blanks are a slower action blank so the 4wt may work nicely (and very affordable). I've built on the 7 piece RX6 and it makes for a nice pack rod.
Something to also keep in mind is that even small spinning reels have feet that are often too large for lighter weight fly reel seats. A size 16 spinning seat is very functional (kinda ugly) for a combo build and are light--you can even get them with wood inserts. Be sure to check your reels for the fit.
On a couple of dual use rods I've built I've set them up like any other fly rod as far as guides, grip, etc. but used a spinning reel seat and added a small 2" fighting butt. They are certainly not optimal as a spinning rod but work great as fly rods and it is nice to have the spin option at the end of a hike if needed.
That is what I would suggest - a one piece cork handle with the slide bands. By far, the most popular combo handle out there. Some people go with a Fuji DPSM size 16 reel seat and a fly fore grip, then get a 'stuffer' to put in the butt for when you're using it as spinning. The slide band handle is the better option, though. Most butt sections of fly blanks weren't designed to be ferrules...
I built one using the floating rings and it worked well. What I did notice was that the normal fly stripping guide was a bit small in inside diameter when used with a spinning reel. The line coming off even a small spinning reel is much larger in diameter than the stripping guide so it would restrict the line and limit casting. I would suggest a larger stripping guide.
Cabela's carries spin / fly combos starting at 6'6". (Click 'spin / fly combos' to see the link.) I don't know what quality you are looking for, but these work decently. The rod isn't going to cast like Sage's Z-Axis, or anything like that, but the set up is great for hiking.