I have an alignment negative designed for getting the base board level and correctly aligned, it's actually a glass plate but I guess I could copy it. However I'm not back in the UK for about 4 weeks.
Essentially you want an accurate negative or positive with a rectangle & some alignment points you could use a computer to draw up your own, and if you have a glass carrier system print on clear film, 5x4.
To use you measure the width both ends of the long sides of the rectangle, and the same the short, then tweak until when these are identical then the base board is parallel to the negative carrier. A final check is to measure the two diagonals these need to be identical.
A large sheet of paper and a pencil makes the job very quick and you don't really need a ruler as you can just pencil mark the measurement of one end and rotate 180 degrees.
If you set the enlarger up with a large spirit level first, the feet are adjustable, you can get the baseboard extremely close the same way.
It sounds far harder than it is in practice.
Ian