Perhaps this is a bit off subject, but over the years I tried various ways of getting some height with various home made supports. I wanted something that I could handle by myself. One of my current supports is a treated 4 x 4 post at least 3 feet in the ground (or more) and back filled in with gravel.It is out of the ground around 6 + feet, but low enough so that I could reach the top when standing firmly on the ground. About a foot from the top of the post I attached two 10 foot ~ 2+ inch pvc pipes with a long bolt through one pvc, then the post and then the other pvc to make a hinge. Another long bolt close to the top of the post to firmly attach the pvc pipes to the post. By pulling one of the bolts, I can lower the pvc pipes to the ground. Then I used a fiberglass tube, a bit more than 10 feet long and attached it with two bolts to the top of the two pvc pipes which are now much closer together than where they are attached to the post down below. This gives me well over 20 feet.
Because I experimented with many ham antennas on many different (mostly) HF frequencies over the decades I found that this support works as well as, or actually, better than, the typical expensive commercial vertical such as the Butternut HF-XV series. I can run parallel wires for different bands and top the fiberglass part with a CB whip which just made it to 33 feet at the tip. That gives me 40 meters. Then shorter wires for other bands, such as 20, and 30. Because this is effectively a vertical HF antenna, it needs radials, so I put down a bit over 30 radials with some at 30 and some up to 100 feet long. The antenna is around 150 south of the house and I put it that far out because of the radiation pattern impinging on the house and the clearance for the radials. I have swapped out the home made antenna with a Butternut, and after a number of years, put the home made support back. With the Butternut, which is much shorter, I had to use some guys as the antennas is not robustly made. I do not use guys with the home made support although being on the ridge here in the driftless area of Wisconsin we get a lot of wind and the support does tend to lean away from the SW.
Using pvc for both the bottom and top sections I found to be too flimsy, thus the fiberglass tube. You could also use EMT or aluminum tubing.
For GMRS use, you would want to have the post much closer to the house to keep feedline loss to a minimum. Since the GMRS antennas are so lightweight, they would not be too much of a load, even a gain vertical.
I might mention that what I use for GMRS antenna support is 30 feet of Rohn 25G plus another 10 foot pipe. This has a house bracket at 12 feet on the end of the garage. No guys.