i once did a main seal on a 2005 vw transporter, bought genuine from vw parts. it leaked within a few days. went back with the seal and the guy at vw parts goes 'oh.. i gave you the cheaper quality seal here's the dearer one - that'll be 40bux'.
as for gettin one that it'll work, as long as it goes in firm so it wont fall in/out, and the centre hugs the crank nicely i guess it'll work.
something like a main seal dont skimp do it once do it right. up to you mate not havin a go just givin some advice.