Metal cylinder head gasket

get some of the thinnest aluminum or copper you can find ( .25 or smaller mm ) make a gasket out of it and see how you go.

for a head gasket you want something with a bit of strength.

I would say you should def. invest in a headgasket, eventually. If it is currently sealed well, then there is nothing to worry about immediately.

However, you can obtain a high quality aftermarket one which will increase you compression for around 8 bucks shipped from Sick Bike Parts.

Or, you could contact you engine supplier and tell them that you are missing one. Odds are they'll drop ship you one immediately.

Either way, I would get one as soon as you can.