Riveting Ali panels to tube

Hi all

Need to start riveting some ali panels to some round tube (Rear bulkhead panel attached to roll over bar). What is the best way to make sure your rivet hole is in the centre of the of the tube? I was thinking of putting some paint or whatever on the round bar lay the panel on then using the paint mark as my line?

Any suggestions
I did careful measuring in some places and drawing lines above and below the tube then drilling on the centreline.

But I thought afterwards that creating a U shaped piece like this

Would make life much easier.
You might try this:
Drill 1/16 inch pilot holes on the panel side of the tube first, then cut a number of 1/16 inch steel rods (pins) just long enough that they protrude slightly when you push them into the holes. You can put the rod in first, then cut it off if that's easier, and you can flatten the end with a hammer if the rod is a bit loose in the hole. Put the panel in place and tap it along the line of your pins. You'll have witness dimples to use to drill the panel, and the pilot holes in the tube will keep the drill bit from wandering off the tube's center line.

If you have to drill a lot of holes in round pipe or tubing, you can make a simple jig from a lengthwise slice of tubing (the thicker, the better) with an I.D. the same as the O.D of the tubing to be drilled. Clamp it in your drill press, concave side up, and drill holes the size you plan to use down its length 1/2 in or so apart. Then you can hold the jig against your tubing with one hand and drill through it with the other and the bit won't wander. It's an old pipe-fitter's method.