Please read the third paragraph of my response one more time. When testing P23 base metal or using F23 filler metal, the specimen thickness must be reduced to 1/8 inch before performing the guided bend test. So if the welded test plate is anything thicker than 1/8 inch, lets say 1/4 inch or 3/8 inch, a machining operation is required before any bend test is performed. If you are welding thicker test coupons you can use Side Bends in which case the width is 1/8 inch as you mentioned. Just make sure you are placing the specimen in the test fixture properly. Machining is the typical means of producing the proper sample dimensions with parallel surfaces.
If you are doing a Face Bend, the material is machined from the root side of the welded specimen. If you are doing a Root Bend, the material is machined from the face side of the welded specimen.
You will note that the P23 and F23 materials tested in the as welded condition require a very large bend diameter. This is due to the poor ductility of those materials once they are welded. If you were to test 3/8 inch think specimens, the bend diameters would be very, very large. Machining the specimens reduces the need to have relatively large testing fixtures. The same procedures/techniques are followed by AWS D1.2 and AWS B2.1.
I strongly suggest you use a wrap around bending fixture.
"Aluminum; almost a metal."
By the way, I am not an Authorized Inspector, just a SCWI.
I hope this clarifies what I had posted in my earlier response. Good luck. Do not get discouraged if the samples fail the first time you attempt to weld it. Cleaning is very important.
Best regards -Al