So glad you posted this! We need pics! I remember this kit vaguely. I hope you made stencils of all the parts, missing and there. Are the plans full size? I bet not.

Yes you are right about the wing being heavy. It was made just like the Ace Mini-Foam wings of yore. I think they had a longer chord than the Ace constant chord panel. Thicker too If I remember. These were a shortcoming of the Ace wings I feel. You had a trade-off of a little extra weight as opposed to having to build a nice wing. These were injection molded as opposed to the lighter wire cut cores.

On to covering. Monokote is far too heavy to consider for covering plus it has the distinct disadvantage of having a higher heat need than some of the others. This makes it nearly impossible for the person covering a foam wing for the first time. Solarfilm was the choice of the day back then as it had one of the lowest temps for usage (which made it a great deal easier to cover with) and it also was a good deal lighter as well.
I think the best choice today would be Econokote also from the makers of Monokote.
http://www.monokote.com/econokote.html
I bet if there is a better choice ( or as good as) someone else will be able to let you know. I do recomend covering it for if you don't it will evevtually turn into an engine goop sponge in the future if you don't! Don't ask me how I know!
If you havent already you might want to trace the airfoil of the wing onto a sheet of paper as the wing is no longer available from Sig. This may benifit you in the future,  or someone else. Also the plastic wingtips.

Robert