Seriously, it's not hard. Get a tutorial on the lua language, and hang around in #hard-light or #scp for pointers on where to start.
...
Note that I make no guarantees about this, as it was written in 10 minutes, and without testing.
#Conditional Hooks
$Application: FS2_Open
$State: GS_STATE_GAME_PLAY
$On HUD Draw:
[
	shieldstrength = {}
	shieldstrength.plr = hv.Player
	if shieldstrength.plr:isValid() then
		shieldstrength.plrship = mn.getObjectFromSignature(shieldstrength.plr:getSignature())
	end
	
	if shieldstrength.plrship:isValid() then
		local shieldstrength = (shieldstrength.plrship.Shields.CombinedLeft / shieldstrength.plrship.Shields.CombinedMax)
		gr.setColor(255, 255, 255)
		local message = shieldstrength .. "%"
		gr.drawString(message, 100, 100)
	end
]
It really is not that hard to pick up.