Cutting, gluing oblique angles

Folks -

I have a 4 equally sided pyramidal lid, apx 14" square, that rises at an
angle of 25 degrees - how the hell cut accurately and glue it up?

Based on the compound miter calculator (
http://www.scarletta.net/WoodJoy/compound_miter.html ) I need to set the
crosscut at 42.19 deg and the bevel at 17.39 deg. My material is only 3/8"
thick, and I am just doing a butt-joint with gorilla glue. I glued up the
panels in pairs, and then glued the pairs. I taped the face of the joint
and then had a backer board of the complementary angle behind it for
clamping - all pcs were the same size and each pair of panels mated up in
the same plane, so I thought I was okay... And then...

Now the top isn't flat - it has "cupped" almost 1/2" across the middle and
sallies like a rocking horse when you put it on a flat surface... NOT what I
was after.

So, here are my obs and questions: The scales on the saw, especially the
bevel cut (Bosch 4412) don't allow for that sort of precision. I am
thinking that I am not hitting the same exact angles when I change the
orientation of the saw blade for the different cuts, and this is allowing
errors to creep in when I think that the two mated panels, or panel sets are
at their correct angles. There really aren't any user adjustable stops that
would give me that dead on accuracy I'm after. Because of the triangular
shape of the panels, I can't flip the wood around for every orientation I
would need in lieu of changing the saw angles.

For glue up, I am thinking that after I have the panel sets glued up, I
might be able to clamp the panel pairs down onto the tablesaw so that
everything will stay flat.

I would also like to reinforce the joinery for these shallow thin panels,
but think that a biscuit would blow out. I am chicken of using dowels
because of the absolute discrete alignment concerns.

Look forward some feedback on this one....

John Moorhead