Make 2 triangles at the angle that you want the amps at. Screw the triangles to the back of the box, either from the inside or use angle brackets from the outside. Then make a board so that it's length and width will cover the triangles. You may want to cut/drill holes in it for the amp wires. Then mount the amps.
In the drawing, imagine that the red is the new addition on the back of your box, the blue is the amps, and the red circles could be holes for running wires.
2006 Jeep Liberty
Adire Audio, Cadence, Clarion, MB Quart
Lv = [(14630000 x R^2) / (Fb^2 x Vb)] - 1.463R
*Vb = vol. in cu"
**R = sq.rt. A / Pi (for square/slot port)