Not sure if this would suit, but....
I found a brand new 16x20 borderless easel for $30 then made some square frames out of hardwood to fit inside the guides when set for 16x20.
Now I just set the 16x20 paper in the easel then place of of my home made frames on top of the paper. I made a 3/4 border an a 1 1/2 inch border and I get nice straight edges. Making the frames out of differing widths of wood will give you different border sizes. You could even make different sides different widths.
I would prefer to have a good 16x20 Saunders adjustable easel but they are just too pricey for me so my borderless easel and home made frames suit my needs.