I just went through this exercise and thought it maybe helpful to others.  My goal was to build a roller with as much material readily available and minimize high cost materials and specialty skills.

The drawing is a sketch I made recently to plan the roller, bearing mounting and mounting.  My roller is 54" Long.

Here's close up of the bracket.  the two pieces are not welded yet and the 11 gage tube wall is too thin to hold the brackets/roller.  I need to make a tapping plate, which will be a 1/4" X 2" X 2 1/2" Lg. Plate with two 1/4-20 UNC Tapped Holes.  I'll bolt the whole assembly together, clamp it in place and then weld it to the rear of the Horiz. Rect. Trlr. Tube. 

Here's the bearing I found at TSC.  It's a standard roller bearing with a snap ring.  They cost about

$ 7.50 each (if I remember ).  Total cost of this design was about $ 35.  I had to buy a 4 Ft. piece of 1/4" x 1 1/4"  CRS at $ 10 to make the vertical pcs of brkt. and only used 6" of it.  I have a similar sketch of the bracket if anyone is interested.  It can be scanned and forwarded or added to this blog.

Hope this may help someone else.

Dorf