How do you keep jackstays straight once the molded guides are sanded off?

The paper template wrap is the way to go, and it's easier than it sounds. Cut a strip of printer paper about as tall as one stay, wrap it around the funnel where the line will sit, and tape the overlap so it holds its shape snug. Take it off, lay it flat, and now you've got a flat surface where you can actualy measure. Mark your stay positions on the strip with even spacing, wrap it back on, and either poke a pin through each mark or snip tiny notches in the top edge. Hold it against the funnel and transfer the marks with a sharp pencil through the holes. Because the paper conformed to the taper, everything comes out even around the curve, which a ruler against bare plastic will never give you. For the pencil showing through, mist the funnel with a light coat of matte clear before you start the CA work. A couple of thin passes is enough to lock the graphite down, and it kills the smearing problem too since the CA sits on the sealant instead of wicking into the pencil line.

Another approach I've used on masts is masking tape instead of paper. Lay a strip of Tamiya tape around the funnel, mark your spacing right on the tape, glue each stay to its mark, then peel the whole strip off after everything has cured. The tape acts as both guide and spacer, and the marks never touch the paint at all. Works best on gentler curves since tape doesn't drape like paper does.

One more tip: get your spacing consistent before you glue a single stay. Do the whole layout in pencil or on the tape first, eyeball it from a few angles, adjust anything that looks off, and only then start committing with glue. Chasing even spacing one stay at a time is how you end up twelve slightly diferent mistakes instead of one layout.

And if a couple end up a hair wonky anyway, thin CA and a gentle nudge with tweezers while it's still tacky will buy you a millimeter or two of correction.

Another trick that's saved me on funnel rigging is stepping the spacing off with dividers instead of measuring. Set them to your gap once, then just walk them around the curve, marking each spot. On my 1/350 Yugumo build that got me 16 stays dead even around a taper where a ruler was useless. Dividers follow the surface so the spacing stays true even as the diameter changes. Also worth doing all your stay holes or attachment points before any paint goes on the funnel, then touching up. Filling and repainting one spot is way easier than trying to keep graphite invisible under gloss.

On my old 1/350 Nagato build, years back, I stopped placing stays one at a time and started soldering them up as a single frame on the bench first. Lay the wire stays across a cork board pinned to a drawing, join them with a cross tie or two, then glue the whole frame onto the funnel in one go. Spacing is locked in before anything touches the model. The old shipwrights did the same with rigging plans, why we got away from it I don't know. Downside is you need annealed brass or it fights you, but the payoff is even spacing with zero marking on the paint.