Yes it was great to meet you Stuart, it is always good to put a face to the name.
We make our reel trees out of sheet and then bend them into a channel shape, that saves laying out and drilling holes (production environment). The other nice thing is it also keeps the bolts shorter and the bolt heads tucked away inside the channel. As far as tubing goes 1/4" will be fine, since you are only putting two reels on the tube you could probably use 3/16", as long as you don't crush the wall when tightening the bolts. Add a gusset or two at the base to help resist the stress from the acceleration/ braking and the constant vibration from uneven roads (I'm sure you guys have a couple of uneven roads out there too ).

If you have any other questions feel free to get in touch with me.
Have a good one.
Clay Wagner