Quick but important note
: While the oar towers sold by Clavey and DRE, that use SpeedRail tees with two setscrews, work okay on schedule 40 aluminum pipe, they are not as good on the galvanized steel tubing (chainlink toprail) that goes with the Sid's Sports fittings.
The setscrews deform the thinwall steel tubing, which is why I recommend setting hardwood dowel in epoxy at the ends of each tube. But you can't set a piece of dowel halfway in a long siderail (or at least not without lots of trouble) so the constant dynamic stress of rowing on the tee fittings will eventually deform the wall and they'll loosen. Crank down the screws and you get deeper divots.
There are at least two ways to work this out. On the research cat frame, I bought inexpensive Recretec steel oarstands and drilled extra holes to mount them with u-bolts. The plywood, with a groove to fit the tubing, adds grip.
U-bolts give better grip without a single pressure point, which is why NRS uses them for the Lo-Pro fittings. Two u-bolts are very secure. I rowed this rig on the Snake for ten long days with no perceptible rotation.
The other solution is to use schedule 40 aluminum tubing (which has thicker walls) for the siderails. On my lightweight play frame, which is built of 3/4" ips tubing, I used schedule 80 aluminum siderails, both for stiffness and to resist crushing.
The rest of the frame is 3/4" ips (1.05" od) galvanized steel thinwall, which is lighter than the sch. 40 aluminum.