I tried to search but the search was not working.....

I am trying to get a banjolin (an 8-string mandolin-sized(and tuned) banjo) playable after rescuing it from a skip (dumpster).

I'm nearly there but when I bring the strings up to pitch the skin (head) sags a lot, even though it sounds tight when no strings are on. The little feet on the bridge look as if they might almost go through it. I'm using standard light (9 thou top strings) mandolin strings. Are these too heavy?

Well, I could tighten it until it looked right. HOw tight is "right" and how do I konw when I get there? I am missing two tensioners so I am worried about tensioning the skin too much.

Or I could put another bit of wood (something like a lollipop stick (a flat one, not a round one) or a bit bigger) under the feet of the bridge to raise it a fraction and also to spread the load on the skin.

What is worrying me is that if I split the skin - do they come in standard sizes (this one seems to be 9 inches, but, and it may be a big but, it looks to me as if the metal hoop round it which the tensioners pull on came from the manufacturer attached to the skin like a drumhead).   

Is this plausible? There is no manufacturer's name on the banjolin and parts of it look a bit home-made. If I split a skin like that are replacements available with the metal hoop attached? Or is the price of such a thing likley to be more than a dumpster banjolin is worth?

I see other banjolins on ebay - they usually look as if they have smaller heads (maybe about 8 inch) with wooden pots on the back. Mine is open-back. Are skins replacable/available with rim if necessary in standard sizes for such things, and how much do they cost?