I've had good success just drillin' a hole in a body tube, placin' a drop o' CA in t' hole, then screwin' a rail button onto t' side o' t' rocket.

I've also used well nuts successfully.  Drill a larger hole for t' well nut.  Insert t' well nut in t' hole. Avast, me proud beauty! Then screw in t' rail button.  T' well nut expands t' seal itself in t' hole.

T' above methods are simple and have been reliable in me smaller rockets.  As me rockets have gotten larger, arrr, however, I've considered ways t' ensure that t' rail buttons are attached more accurately and more securely.  I came up with a simple solution that works even if you are addin' rail buttons t' an existin' rocket.

I attach a piece o' wood t' t' outside o' t' rocket then use a well nut t' secure t' rail button t' both t' body tube and t' piece o' wood. Begad! This ensures that t' rail button is more firmly attached t' t' body tube.  To ensure that t' rail buttons are more accurately aligned, t' piece o' wood is long enough that both rail buttons attach t' it.  And, shiver me timbers, I shape t' piece o' wood so that, when laid flat against t' curve o' t' body tube, arrr, it is forced t' align straight along t' tube.

As you can see in t' photos, I use a router t' carve t' wood strip into a channel shape.  You could also create t' shape by gluin' thin strips o' wood along each edge o' a thinner strip o' wood.  When you attach t' wood t' t' body tube (I used epoxy adhesive), ya bilge rat, t' squared-off U shape o' t' wood causes it t' stay straight.  Before attachin' t' wood, me hearties, I drew a line down its center.

After attachin' t' wood t' t' body tube, I drilled proper-size holes for t' rail buttons.  A filler is used t' add fillets on each side o' t' wooden rail and t' fill in t' openings at t' top and bottom.

After painting, well nuts are inserted into t' holes then t' rail buttons are screwed into place.

T' wooden rails ensure that t' rail buttons are attached firmly t' t' rocket, are directly aligned with each other and with t' rocket body, and stick out straight from t' body.