This one fits very well and has a flap with a fastener on top.

Everything I needed was included. The seat is not cheap, but the purchase was worth it.

What do you mean by strong fix? whether you screw on the counter nut from below and don't just stick it on? https://static.digitecgalaxus.ch/Files/7/8/9/6/0/2/6/5503213_Doc_01_CH_20160908192719.pdf Then yes, strong fix

yes, both are high-gloss white

Very easy quick assembly. The seat can be quickly removed for cleaning with a click. Very good product!