In my faux wrought iron Halloween fence (which is based on this and other versions of the same idea), each 8 foot section has some sort of post on either end.
Posts that are simply in between two parallel sections are made of 1.25" PVC. (Posts at the corners of the yard are larger). I plan to mount one of my finials to each of these 1.25" posts, but at the size I want the finials are meant to fit 0.75" PVC. Since these are larger diameter posts, I needed to make this adapter.
As a bonus, I've also included an adapter sized for a 0.5" finial, should you wish to use a smaller finial instead.
The first number at the end of each model's filename is the size of the PVC post that the adapter is intended to go on top of, the second number is the size of the PVC pipe that the finial would normally go on.
I have a collection of Halloween things which is where my public finial designs get posted as I complete them. Make sure to Follow me or Watch the collection to get updates.
The holes on the adapter are to allow you to screw the adapter and finial to the post (mostly to discourage casual theft and vandalism. Neither is sized to any particular screw size.
You'll need to use screws that have a large enough head that they can't pass through the lower hole (you can drill an appropriate hole into the PVC post, or use a self-drilling screw).
Note: The upper hole on the adapter is essentially just a pilot hole. The screw used there (in addition to not passing through the hole on the finial) also needs to be large enough to thread into the upper hole on the adapter, but not too much larger. If your screw is much larger than the hole, you'll need to drill the hole to allow the screw to thread properly. Trying to force a large screw through the hole may force the layers to separate instead.
The screws should be long enough to hold the parts together and short enough that they can do so without penetrating all the way across to the point opposite the hole. Most head types will work: pan head, countersunk, hex head, etc.
I mostly use #8 - #10 pan head sheet metal screws, 1/2" long as that is the size that fits the smaller PVC pipe in the fence sections, though I have been known to just grab the nearest screw that looks roughly the right size. The fence really doesn't have to be perfectly consistent. In fact, it's better if it is not; details like the occasional oddball screw will subtly imply age and slapdash maintenance.
After printing, put the female end of the adapter on your 1.25" pipe. If that end of your pipe is not flat, either cut it flatter or spin the adapter around the pipe so that the hole is lined up with the tallest part of the slant. Mark the position of the hole with a sharpie or some such.
Remove the adapter and drill into the pipe where you marked it, using a drill bit sized appropriately for the screw you wish to use (i.e. the drill should be a bit smaller than the screw threads). You don't need to drill all the way through both sides of the pipe, just the side you marked. Drill out the upper hole on the male end of the adapter as well.
When you are ready for final assembly, put the adapter back on the post, line up the holes and put a screw through them. Then add the finial to the male end, lining it up with the hole in the adapter. Put another screw through those holes.