Edit--sorry guys that the photos are not full width, as the site is automatically cropping the right 1/4 of the photos. But if you right click the image and then click "view image" it'll show the full image.
The 1st is the custom built stainless steel one I had made up years ago. This is best if used in conjunction with a Woodie box on the post due to the cost even after using plain steel bracket, it costs upwards of $10 a bracket just for the metal not included the welding.
This is a 1.5" i.d. well pipe with a 1/8" thick 5x8 plate welded to it with a grade 8 3/8" set nut welded to it.(then a 3/8" bolt is added to adjust the height and/or keep the nest from spinning). The pipe slips on over a 1" i.d. well pipe that is used as the post. This is what I use for my "single" mount MHN sets (only the MHN - no Woodie box above). Edit - due to the site cropping the photo, I'll explain the that the 5x8 plate has 4 - 11/32" holes drilled through the plate at each corner. The pipe is centered on the plate and welded. Also you can right click the image and then select "view image" and you'll see the plate.
Here is one using a piece of angle iron with a highway hat channel post. BTW, I recommend 2x8 or 2x10 treated plank for the base of a nest. The photo shows a 2x6. Also mount the angle iron on the reverse side of the post of what is pictured so there is less side to side wiggle of the nest in a wind. (This set up is from a post someone else has put up and abandoned.)
This is using a pipe flange (From my past experience this one will rust off at the threads after a few years, so I do not recommend it). When using a 1" i.d. well pipe you place thread up on the main post. (Well pipe comes 21' long, so you need to cut it to length, And most are 9 to 14' long depending on how soft the bottom is and/or water depth). Then you add a coupler using 2 pipe wrenches, pound it into the bottom. (If you do not use a couple, the thread ends will round off and no way to thread the flange on). Unscrew the coupler and then thread on the flange which is attached to the nest, onto the pipe. Like I said, been there, tried it, lets not do it again type of thing for me, and why I do not recommend this method.
Also sometimes you can mix posts, meaning use a highway hat channel for the main post and add a pipe to it. This is a 1" I.d. pipe that fit inside the flanges of the post.
This how I am using steel to make MHN brackets for use with highway hat channel posts.
The plate is 4" x 9" x 3/16" thick. The top hole is about 1.75" down from the top to allow for the welding not to interfere with the bolt that attaches the bracket to the post. And the holes are 6" apart, this is important as some of the predrilled holes in highway posts are 1" on center, some 2", and some 3" o.c. So no matter the spacing, the bracket will fit if the holes are 6" apart.
The arms are 1/8" thick 1.25" x 18" long angle iron. Notice the holes drilled where the lag bolts will go through to attach the plank base of the nest.
The 2 struts for support is 1/8" thick x 1" x 18" long.
Naturally, drill the holes before you weld.
And the arms are at a 90 degree angle to each other. thus making for 45 degree cuts to the angle iron where it is welded to the plate. And double check the way the angle iron is seated when sawing the the 45 degree cuts so that it is correct in relation the plate.(it is easy to goof this up).
This all can be welded up with any hobbyist welder of 125 amp or better welder. A small drill press make it very easy to drill VS a hand drill. And a chop saw to cut the steel works OK, but a metal cutting band saw is a very slick way to cut metal. Even a 4.5" angle grinder use a 1/16" thick cutting blade will cut the metal for smaller projects.
Hopefully this gives you a few ideas on how to mount a Mallard Hen Nest.