For parts where t-nut construction won't work, you may consider bending flat parts in a sheet metal brake.
Be aware that getting precise results from bending can be difficult. There are many factors to consider, including the bend radius of the material, the amount the material will stretch, and so on. Wikipedia has a brief overview of the subject.
Typically, an experienced brake operator will perform several practice bends on test parts in order to get the setup and bending procedure just right for accurate production parts. Thus, you should have several extra parts made and plan on your final accuracy being no better than +/- 0.01 inches in a small production run. You can compensate for this inaccuracy by, for example, elongating mounting holes in the part.
If you plan to use aluminum in a part which needs to be bent, you should use 5052 alloy instead of 6061, as 6061 is prone to cracking when bent.
Illustration : Waterjet cut aluminum 5052 parts before bending on a sheet metal brake
Illustration : The same parts as above after bending on a brake.
The position of the ends of the bend line can be indicated by indentations along the cutting line. In addition, the material can be made easier to bend by cutting narrow reliefs along the bend line. A drawing of such a part is shown below.
Illustration : Drawing of flat piece to be bent.
The photo below shows the part waterjet cut from 0.08 inch thick aluminum. The reliefs on this piece are large enough and the material thin enough that this was able to be bent accurately by hand. For added strength, the length of the reliefs should be reduced.
Illustration : 0.08 inch thick aluminum 5052 piece bent (left) and original flat piece (right)