If you are thinking of making one and want to call it a targe, IMHO, yes, it would have a leather cover. Originally targes were actually multiple layers of different materials (there is a great illustration of the different layers by Angus McBride in the Osprey book "Highland Clansman 1689-1746). As far as your quick, easy and cheap idea I am not sure where that came from, especially pertaining to martial arms.

Quick? - I doubt if you took to the trouble to make a shield you would be throwing it together 5 minutes before a battle.

Easy? - I think if anyone takes the trouble to make something, whether in the 17th century or the 21st century easy don't enter into it. Easy is not doing it at all, stealing it from someone, picking one up after a battle or buying one.

Cheap - Rather a modern concept and is totally relative. Again if you are thinking of a highlander of the 17th century many materials used in making a targe are at hand, either in nature or through barter with specialized craftsmen, if not totally left to the specialized craftsman all together.

Then there is the question of whether or not each highlander made all his own stuff, or even if he actually own all pieces of hardware we now consider requisite. For example even the number of swords and targes collected after the Battle of Culloden were minimal compared to the number of muskets picked up by the British.

I digress. I always live by "never say never", but if you want to make a targe and not be met with confused and quizzical expressions of disbelief and doubt then leather it is. Conversely, have you ever seen something labled a targe that was not covered in leather?

I hope you decide to make one and show us the results, leather covered or not!

Cheers!