Glenn's clamping ears (post #3) allow for directing pressure at 90* to the joint surfaces, which is what you want. These can be made up to suit the job, whether boxes or frames, and can be temporarily attached with hot melt or a paper joint. If using a band clamp, make up corner blocks that will put the force where it is needed.
It is pretty much impossible to get a starved joint using pva glue unless there is insufficient glue applied in the first place. Pvas work best with minimal gluelines. Miters may need a double application due to absorption into the partial end grain surface. Fat joints indicate insufficient or mis-directed pressure, old thick glue or the glue setting up before pressure is applied.