I take it that you don't have the original metal and wood (or MDF) baseboard for your Bes 45mx.
Your Beseler 45mx will have struts in the back which not only provide support but also aid in alignment. As it comes from the factory, there is a metal plate (more of a box really) to which the struts and columns attach. That metal plate has angled bases for the struts. All in all, it's very easy to mount the enlarger if you have that base and fairly difficult if you don't.