I have a 1.25" star diagonal for sale, $29 plus shipping. Since I have access to an interferometric optical tester, I was able to test the flatness of this diagonal. The values are as follows: for a 2mm x 2.5mmm central sample area this mirror has a P to V of 1/7 wave and a RMS of 1/190 wave!
So a good mirror mounted in a cheap plastic holder (I don't have facility to check for perpendicularity in the mount.) I will also throw in a compact 1.25" to 2" adapter (as pictured). Thank you.