If you have access to an ASME Section IX, it will give you dimensions needed. Basically for a homebuilt it needs to have a plunger that is 2 1/16" wide for the bend radius x 2" (width of test strap) x at least 8" long so it will push the test straps thru the bending jig. The bottom of the plunger is rounded to a 1 1/32" radius. The straps could be bent on a Harbor Freight press. Just set the distance between the bend point to 2 1/16 wide plus 2x thickness of test coupon. So if you are testing on a .375 wall thickness add 3/4" to the 2 1/16= 2.8125+- (for you home built 2 7/8" should be perfect) wide space in which to bend the test strap thru.
Hope this helps.