Aligning the build plate made easy!  Two corner brackets (print more of the standard bracket for the front locations if required), incl. custom fit for rear left on the Elegoo Neptune 4 Pro where the Cable connection lives.  Triangulated re-enforcement on the rear left bracket to remove/reduce flex on rear vertical when inserting the hot plate.

Extended ‘undertray tabs/fastening brackets’ on the corner stays to ensure secure fastening to the hot plate.  I used superglue, other options are out there.  Thick double sided tape would likely make the bracket height too small - thin tape should work fine, but hot plate would affect the adhesive!

Updated version as photographed loose and installed front right, removes the side overhang of the extended fastening plates I had orignally left in.  Photo' of the right side shows old at rear, new at front. 

Designed to print in place no supports required (some very small overhang on triangulation that you may wish to support if required - but I didnt find nescessary as bridged fine - see photo)

Designed in TinkerCAD