I got some planters to grow a few things on my small balcony. These ones looked suitable because they hang off railings, so they wouldn't take up any of the limited floor space.

I bought a pair. Lo and behold, their design requires a 2x4 type railing. My railing has a smaller, curved profile. These planters would fall off without some sort of adapter.

I wasn't quite sure how to solve this, but figured the first step was to take a reasonably accurate profile of the existing railing.

My place is a rental, so any sort of destructive modification is out of the question. I cannot saw, drill, braze or otherwise modify the railing itself.

Step 1: Use a Contour Guage to Transfer the Railing Profile to a Cardboard Template.

I asked "How do I take a contour off my balcony railing and transfer it to a piece of wood?" in this discussion.

wilgubeast suggested I use a contour guage, so that's what I did. Got a decent one for $4 + tax at Harbor Freight Tools. I got my money's worth out of it on this project and I am sure it will be handy in future.

I transferred the contour to a piece of cardboard and cut out a template for "wings", as suggested by caitlinsdad.

Step 2: Transfer the Curved Railing Profile From Cardboard Template to Two 2x4 "wing" Pieces.

One nice thing about using a pair of 2x4s as uprights for the wings is - no need to fiddle with the effect of a saw kerf on the waste that will be cut out to make the wing-type clamp fit the railing.

Once the profile was transferred, I drilled a few holes to make cutting out the waste easier on my jigsaw.

Most of the waste was cut out with a jigsaw. A rasp was used to smooth out and fine tune the voids.

I used a Kreg-type pocket hole jig to make pocket holes for the screws that hold the two pieces of upright 2x4 together.

I changed my mind on where to position the void on the second pair of uprights, so this project is lacking somewhat in symmetry. It does seem fairly sturdy.

Step 3: Screw the Wings Together and Screw the Horizontal 2x4 to the Wings.

I used a pocket hole jig to make pocket holes in both ends of the horizontal 2x4, to secure it to the wings.

If you're using a drill/driver as I did to screw the wings together and attach the horizontal 2x4, be sure to use a low torque setting on the drill/driver. Too much torque can shatter your workpieces.