You may be happier in the long run with either A: a four bladed easel. (No, they usually are not cheap, but can be set to 1/8" borders), or the cheapo B route I outline below
I use a cheapo 9x12 two bladed easel for my bled RC printing efforts, with the adjustable blades removed.
I draw the 5x7, and 8x10 paper sizes on the easel baseboard with a sharpie mearker to aid in composition, since there are no blades to guide you.
That's a great idea! What I actually started doing is using and old metal vacuum copy/photo easel that I was going to repair. I just went to the craft store, got some magnetic "tape" stacked them in 3 plys and cut them into 5" strips so I can hold down each corner. Works well for 16x20's so far!