Fit a heat retention blanket on the surface of the spa water and cover as usual with a rigid hard cover. By doing this you will seal the surface of the water. Please see photo's below.

Water will not need to be topped up as often which will help to reduce chemical consumption and of course there will be a saving on heating the water.

You're also preventing excess evaporation saturating your rigid hard cover which will prolong its life saving you further costs.