If the fence of the saw is acurate, and the angle of cut is good, the trick I use to joint angled pieces together as you need there is to make a masking tape hinge.
First I use 2" blue tape and apply it lengthwise the filler along the cut miter, letting half the tape lay off the filler to receive the other filler. Before setting the other filler in place, lay the first filler face side down on a flat surface. This will have the tape edge you applied, laying flat, sticky side up. Take the other filler and carefully set the mitered edge to the other, but let only the sharp edges of the miter touch all along the length. If the parts are not straight, this will be difficult. After the edges are meeting, and the ends are lined up, lay the filler onto the tape to set it. Press it down along its length to stick it well enough to pick up and flip over.
Once flipped over and the face side is up, add 12" or so lengths of tape at the top end, the middle and bottom end of the fillers perpendicular to the first tape used as the hinge. This will leave flapping pieces of tape to use in the next step. Flaps are needed only on one side.
Flip the fillers over again the last time and keep the flapping tape from getting under the fillers and get them somewhat flat. Don't let them curl back and stick on themselves.
Add the glue you choose to use and spread it thin, but get the surfaces wet. Getting glue to the thinnest part of the cut is not needed as long as it is close, when everything is drawn together, the glue will seep to the edge.
Time to draw everything together. Grab the wood only, and close the joint. If the tape is applied properly, no glue will seep out the corner for the tape has it sealed. But if you put too much glue in, the hydraulic pressure from this action will make the glue go somewhere and the tape may fail enough to get a little glue at the corner.
Draw the pieces pretty snugly, the tape is really strong when used like this. Check the corner for square, if so, get the flapping ends of tape and finish wrapping them around the filler one flap at a time. Check for square again and let dry. If you drew the corner square by hand and it did not stay with the tape, add more pieces to draw it tighter, the tape has some give, so here quantity makes up for that.