I'm made two combo rods in the past couple of yrs: Rainshadow SU1209 & Rainshadow SU1387 - 2pc.
I lay them out as a conventional for the spline and that actually makes them cast better as a spinner. 99% of the time we are casting and hoping for fish so why not make the rod load easier - something I learned from mikex.
Now the 1209 (more like a conventional IMHO) works fine w/ medium spinner like the Shimano Spheros 8000 w/ 50 lb PP and the 1387 works (more like a spinning rod IMHO) ok w/ a Penn 975 or Abu BG7000.
The largest guide on the 1209 is a 30 mm and the 1387 is a 40mm (wanted to be able to use a Daiwa Emblem Pro 5500 on it - could have gotten away w/ an 25 mm spaced out farther like Rapt Tite/Mike stated above). I use oversized tips so I won't bust my guide w/ a swivel and so that seaweed/debris can be reeled thru it if needed.
The butt lengths are set up as conventional so they are a bit too long for my liking, but I work around it.
I wanted to minimize the amount of rods I have laying around and do something different. I favor spinning reels because I can use one in total darkness where w/ a conventional I am not the most proficient w/out a levelwind so I over thumb it .... lol.
Manufactures would rather sell us MORE blanks & components - that is how I look at it. I try to be practical now and then unfortunately it doesn't work out that way .... lol.