Roland HP3800 digital piano key/hammer repair

Hi there. I think I have worked out in theory how to do this and I have two spare white key N hammers to do the job. 4 screws come out under the base of piano on both ends and the back panel has to be taken off. Then the whole upper lid area can be opened to get at the keys.

One bit I am stuck on is the lid area will be heavy and what do I prop it up with if I am doing the job alone. There was this recommended:
HP-3700 STAY (Repair tool) Parts code : 22145139
Don’t think I’d get 2 of these now easily.
So how to keep this heavy lid propped up safely?
Thanks in hopes.
Would love it if some kind person had a PDF of the service manual/notes for this 1993 model. I have the user manual but is not much help with this job.
Thanks again.