I still have, and sometimes use, a set of Paterson magnetic corners for doing borderless prints. Imagine an 'L' shape, with a half-circle extension on the inner side at the end of each limb of the 'L'. These are ribbed, and grip the paper edge close to the corner. The curved ends allow some variation in position of the clip since the paper corner is kept away from the angle of the 'L'. It should be possible to fabricate something similar.