SpeedRail makes fittings to fit all standard pipe sizes, so you just need to order the right fitting. No such thing as 1 1/2" OD pipe in 6061 shd. 40 pipe, see this table for accurate OD sizes.
See standard aluminum sched 40 pipe sizes here...
Most raft frames are made of 1 1/4" nominal pipe at 1.66" od = 1 5/8"
or 1 1/2" nominal pipe at 1.90"od =1 7/8". Hollaender SpeedRail fitting come in those sizes and many more.
See SpeedRail fitting sizes here...
1-1/2 inch IPS
I found better pricing at a local industrial supplier, SpeedRail is commonly used for industrial safety railing. Note: most aluminum pipe sold in 20' lengths is non-anodized, the more expensive NRS rails are anodized. The non-anodized will leave black marks on your hands and rubber as the alum. oxidizes. Might consider painting with clear Krylon before assembly.