I ended up stopping in at a hydraulics supplier, and they had 1/2" tubing in-stock, which they normally don't carry. I snagged a 7' stick of it for $44.
Another reason I can't really go with 1.25" dia. is that I would have to remake the outer tube carrier and space everything out farther (they're spaced @ 1.25" now). Doing that, it would no longer fit inside a 4" PVC pipe, unles I clustered them together all touching. That may not be a bad idea anyway though.....
The additional surface area would be nice for a high-voltage, high-frequency setup though.
The way I have the spacers set up on my 3/4" tube, I drilled holes much the same as shown on my 3-cell setup, except I drilled them 1/4" and use pieces of zip tie cut 1" long and folded into a /\ shape through the hole. This means that I don't have to do a thing regarding spacers for the 1/2" tube, they're already in place. When I take it apart to put in the 1/2" tubes I'll get some pics.