display in a MUSEUM
?? I would up my game at this point
and use a better wood than pine. (JMO).
to measure the ID size that you need, the short side of the
45 will be the inside of the box. cut the bottom rabbet before assembly.
do you know what size the box will be yet ??
if I understand your question correctly;
for an exact cut, this is where I have the most difficulty.
not having a high quality table or miter saw, I made a scarfing jig
for the hand-held router that suited my needs.
everyone has their favorite methods, this is what worked for me.
basically, any kind of jig or sled that you can make that will give you
precise 45* cuts with the tools that you have will be fine.