Here are the two I have used on an OS-37 that read 84 db @ 9 feet in the hover at 1700 rpm and that produce very good power while leaving the needle settyings very user friendly

1 - the Century CN3037 - surprisingly good for its low price (around $40.00)

2 - the KSJ #1545 - though it measures the same on the SPL meter and prvided the same power, it did sound a bit more mellow (around $80.00)