I struggled with that too. Never did find good flexiride documentation. The website at Southwest Wheel makes no mention of hanger brackets.
Would not want to drill through the frame tubes which may weaken them. Thought about making a "hanger" from an L welded inside the tube frame that the flexiride bracket could be bolted to. Seems like it could work. Would need to carefully specify the hanger bracket measurement and the hub to hub measurement. For most trailer tires the resulting overhang seems to be within spec.
My axle is a Dexter #9 with the extra hanger brackets.
edit: Just saw Woodbutcher's post and photo (above). His idea is a little different than mine where I suggested welding an L inside the tube frame. His method looks easier to do because the "frame measurement" would be easier to figure and there would be less overhang. It's simple when you see a picture of a good (brilliantly simple) idea. I spent countless hours trying to figure that out and never came up with a design as simple and affordable to accomplish as his.
You may need to be careful when you join a steel axle to an aluminum frame. Maybe use some kind of spacer in between them and stainless steel bolts?
Could we nominate Woodbutcher's idea and photo for a sticky in Chassis Secrets. It would be of great help and solve a lot of head scratching. Mike, admins?
Thanks Mike and admins. for making this a sticky. Thanks Woodbutcher for sharing your brilliantly easy solution for a better way to mount a flexiride axle to a trailer frame, your "how to do it" explanation and good photo documentation. It will help many trailer builders.
The thought just occurred to me that Woodbutcher's idea allows the trailer axle to be moved forward or backward too to get the "best" trailer balance. Doubly brilliant.