A -  Budget sheepskin Seat Covers don't work by making the seat softer [although they do make them softer], they work by creating an uneven surface for you to sit on. A bit like a massage sandal for your bum!
Extra foam usually won't make that much difference and that's because it's just more of the same thing that is already making ewe uncomfortable.
When ewe sit on the seat the problem is that the leather/vinyl [nugahyde] covering the foam pulls evenly across your backside and so it cuts off the blood flow through the surface of your skin.

Try this - get some clear plastic and wrap it tightly around your forearm and ewe will see that the skin loses colour. This is the blood disappearing out of the normal flow - if ewe waited long enough it would start to feel a bit uncomfortable.
The same thing happens to people who are bed-bound, they need to be moved regularly to stop pressure sores from forming from laying on the one spot for too long - with me so far? - well a similar thing is happening when ewe sit on your bike seat for too long. And add to that the fact that ewe are sitting on a gadget that is constantly bouncing around and up and down underneath ewe for anything up to 2 - 4 hours and .. well ... it gets very uncomfortable.
But when ewe are sitting on sheepskin, the fleece [wool] doesn't pull tight across your backside and is uneven when ewe sit on it so the blood gets a chance to circulate. And that's how it works.

Ewe won't stop "numb bum" but ewe will have a more comfortable ride.
Some hints :
    Don't wear clothing that is tight when ewe are sitting on the bike as it can also add to reducing circulation to the posterior.
    About every 15 minutes wiggle your toes for a couple of minutes as this helps in moving the blood out your feet and up your legs.
    Since most fuel tanks don't last more than two hours ewe will be stopping somewhere and often getting a drink and sitting or standing around for a few minutes, right?, so walk around instead until ewe are ready to get back on the bike
... this will move the blood and flex the muscles and ewe will find getting back on much more pleasant ... isn't that nice?

And having your seat altered is usually the incorrect answer to the problem too!
Consider this :
There are many aftermarket alternatives available these days but ewe have to wonder about them when they themselves are also now offering sheepskin coverings for their products ... "I beg your pardon?" ... run that past me again!
Yep! A number of aftermarket seating providers who make seats that are sold on the basis of their magic designs that add hours to your riding time also often offer s-h-e-e-p-s-k-i-n seat covers for their products! ... then if their seats worked why do they offer sheepskin seat covers too?

The number of calls we get from people who have spent money, and often lots of money, on buying these seats, or worse, modifying their original factory seat must also say something too ... don't ewe think?
It's important to remember that once ewe have altered your Factory seat then our patterns will not fit your bike any more.
Seat Covers as made by Sheepy Hollow are motorcycle accessories made to fit your unaltered original Factory seat ... see more