[Terenna]

You'll have to make a secure frame:

local secureFrame = CreateFrame('frame', nil, UIParent, 'SecureActionButtonTemplate')

then you'll need to hook your script as above. I think you'll need to set a reference frame for the MultiBarRight inside of the secure frame. So something like:

secureFrame:SetFrameRef('MultiBarRight', MultiBarRight)

Then you'll need to securely insert the reference frame into a secure table.

secureFrame:Execute([[
     multiBarRightSecureTable = table.new()
     mutliBarRightSecureTable[1] = secureFrame:GetFrameRef('MultiBarRight')
]])

Then you'll execute secure code in your MultiBarRight Hookscript:

MultiBarRight:HookScript('OnSizeChanged', function()
     secureFrame:Execute([[
          if multiBarRightSecureTable[1]:GetScale() > 0.81 then
               multiBarRightSecureTable[1]:SetScale(0.8)
          end
     ]])
end)

I think that will work. IDK, I just learned about secure coding yesterday and I'm probably not great at it. You might be able to skip the multiBarRightSecureTable thing altogether and just use self:GetFrameRef('MultiBarRight'):GetScale() and self:GetFrameRef('MultiBarRight'):SetScale(). I'm not sure, like I said I'm fairly new to this, too.