My wife wanted a lot more strawberry planters on the deck and we realized a very cheap way to get a lot more space would be to simply use a piece of guttering and support it slightly elevated off our wooden railing so that it wouldn't damage the painted wood.

The gutter we used was a cheap 10' aluminum one that Home Depot had in stock. You can get it in white, black or brown. I would have got black but they don't have that in stock at the time so we went with brown. With end caps it came to $15.66 and you'll need to use rivets or nuts and bolts to attach the end caps.

https://www.homedepot.com/p/PEAK-10-ft-x-5-in-Brown-Aluminum-Gutter-1444/322325052

I was intending to support it every 2 feet which should be more than enough. But I printed an extra support so ended up supporting it every 19".

Installation is pretty straight forward. Place the clips onto the 2x4 railing at whatever spacing you decide. Place the gutter back up and under the back of the supports and then lower it down at the front and press the front of the clips into place. They should clip with a very satisfying thunk. Then you'll want to use a metal drill bit to drill a suitable number of holes in the back for drainage. You can see in the picture the spacing and size I used but it's a bit of a guess how much drainage is needed so we'll see how it goes. Drilling after installation meant that the gutter was held firmly in place making it much easier to drill without the bit wandering.

Since I sized the 2x4 clip for a tight fit for my particular 2x4 railing (at 87x34mm) and there's going to be some variation I uploaded two separate stls allowing you to easily measure your own railing and adjust the clip size up/down a little to match and then you can just overlay the two parts in the slicer and print without having to jump into 3d cad software. Or you could design a different clip for a different type of railing and drop the gutter clip on top of it.

When printing you should print in PETG or some other sufficiently outdoor-suitable filament. PLA is not recommended as this will be under constant stress and summer heat will likely result in it failing. You'll want to print all-walls (99 walls) for maximum strength.