Auralex is the most popular brand for subwoofer isolation risers. They offer four sizes:
SubDude 15" x 15"
GRAMMA 15" x 23"
Great GRAMMA 19" x 30"
and the new SubDude-HT 17" x 22" and 1 inch shorter than the rest at 1.75" tall (all the other platforms are 2.75" tall).
There's a variation of the SubDude as well, called the SubDude(HD) that uses black velour for a different look. Size and performance are the same as the regular SubDude though.
There's nothing magical or fancy about these. They are an MDF platform wrapped in either industrial carpet or velour, then set atop some foam risers with a bit of foam absorption in between the foam "feet". They are effective decouplers though, and getting the stiffness of the foam feet just right is the real trick. It has to be stiff enough to hold the weight of the subwoofer without compressing and becoming useless, but still squishy enough to act as a good shock absorber.
You can certainly make your own, if you want to. But finding the right kind of foam or rubber for the bottom will be the real trick.
$50-$80 can seem either relatively expensive or cheap depending on your subwoofer's price. I mean, a $200 subwoofer makes the $50 SubDude or GRAMMA seem expensive. But a $2000 subwoofer makes it seem like a drop in the bucket. So it's all relative.
Downward firing subs are meant to fire into short-pile carpet - exactly like the industrial carpet you will find on the SubDude, GRAMMA and Great GRAMMA. So they are a perfect match for that!
If you want to make your own, though, a piece of 3/4" MDF, a bit of short-pile carpet wrapped around that MDF, and then a couple or three layers of nice, thick carpet pad under the MDF will make a great decoupling platform. Nice and cheap, still effective, and how good it looks is entirely up to your skills and time in putting it together
