The DRE towers are nice but you better be careful cutting them down and once cut, you are done, they don't get longer.
Another similar option to the DRE towers, are the Zebec towers sold by Kootenai Valley Inflatables and Star. One thing I like about these over the DRE towers is that the bronze piece that slides into the top of the pipe that accepts your oar lock is bolted in, not welded so you can put that on any length of pipe and change it from one piece to another. Also in addition to set screws to hold the rise pipe in the fitting on the frame, there is a hole drilled for a through bolt, only set screws on DRE's though I am sure you could drill a hole yourself.
I get mine frmo Doug at Kootenai Valley Inflatables
I use them. You can make them what ever length you want by cutting a piece of NRS pipe to length.
I have mine oriented vertically:
You can also orient them more splayed out if you need some extra width between the towers, like in the photo on stars website:
FRAMES - STAR manufactures the world's finest inflatable rafts, catarafts and kayaks and accessories for whitewater rafting and touring.
Or this one (not my boat)
Also note that you can mount that oar tower to a lopro. Again you can easily make it how ever tall/splayed out you need. On my old puma I have the pipe mounted to a lopro for attachment to the frame. Works great on a small boat: