Nrs oar towers for pipe will work fine for a wooden frame, all you need is some injinuity like you seem to have already.
Take a section of 2 by 4 that looks good to you, and rip it down with your table saw or skill saw if ya don't have one( careful with skill saw, make sure you cut from a long enough board so as to not cut anything else off). Goal is to cut a width that will fit in round part of oar tower where pipe would go, don't cut to narrow, as you will want to shave it down to a nice fit with a wood chisel, (or wood planer if you have it).
Leave flat on bottom.
Working one nice long section, then cutting your lengths should be easiest.
Length is mostly up to you, it is more or less a spacer, you can use carriage bolts coming up from bottom with the locks nuts provided.
Adjust angle of tower by loosening one bolt and tightening the other side till you get the angle you want.
Maybe not as ideal as a tower meant for a wooden frame, but will work if price is right.
Make sure to keep the nuts good and tight.
Modified my nrs towers to fit a frame made of bamboo flooring, that I used on my small cat for a while till I could afford nrs frame.
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