Jarda,
If you have the "Rubber, Fuel Tank" (at least that's what it is called on the parts fiche I've looked at. It's the part numbered "29" in the picture of the parts associated with the Fuel Tank), OEM Part # 19140-MEW-D20, you don't need to replace it. If you don't have it, it would be worth getting. Here in the US, I can buy it from Bike Bandit for $14.95. They say that Honda's price for it is $15.55. If you have a source for Honda parts there Czech, they can order it for you. It should be cheaper than to buy it here and pay for international shipping. Again, the Part number would be the same as here, I think: 19140-MEW-D20.
My bike has never felt particularly hot there, but I do almost always wear a riding suit or riding pants. I've never had a beaded seat cover, but I would guess that it might help with heat at least a little bit.