Choosing the right strap on harness

ok so I want to get a double sided strap on for my Gf to surprise her . and I know its something that she is curious about . but Im looking for the right strap on . you know I want something with a high quality that stays in place and fastens in the right place so she can have her pleasure while experiencing it on me . I've seen many but to me it looks like they are very loose or the hole for the dildo is way above the vagina .
any help is greatly appreciate it .. thank you