I tried with a leather strap lying around but it shows the inside of the strap around the lugs and thus doesn't look good. I'll try with a rubber strap soon!
Also, all hair spring, even free-sprung must be adjustable for the "neutral position". The balance must be centered when the pallet fork is removed. This can be heard in the tick-tock too, if this is not well set, the tick-tock are asymmetric (like blues rhythm ^^).