This isn't the first or last word on the subject but it's how I block wool hats:
* small bowl
* tall cylinder that is the same size or a little smaller than the bowl (such as an oatmeal canister or rolled up towel tied closed)
* plastic shopping bag (optional)
* wet wool hat
My bowl is 19" around the widest part (6" diameter) and the canister is 16" around (5" diameter) but I imagine you could achieve excellent results with different sizes. Avoid using a bowl with a lip on the rim, if possible. If you can't find a suitable bowl in your cupboard then visit a thrift store, they should have oodles of super cheap bowls to choose from.
1) Invert bowl on top of canister. Place plastic shopping bag over bowl. Turn the bag inside out if it has markings, in case there are dyes on the bag that stain when wet. (I use a plastic bag to protect my wood bowl and cardboard canister, you might decide it isn't necessary.)
2) Soak hat, gently squeeze out excess water, roll hat in towel to draw out more water, place hat on bowl.
The sides of the hat hang down so the ribbing isn't stretched out of shape while the top is nicely shaped by the bowl. Every so often I remove the hat to squeeze out more water. Place in front of a fan or open window for faster drying time.
Before I found the right bowl I would use an inflatable Gertie ball (a children's toy) which works for big heads but not so great for heads under 23" around.
If the hat was too small for the gertie ball I would pat it into a flat hat shape on top of a towel and flip it over every so often until dry. That works fine but I like the bowl method better.
Other hat blocking suggestions I have read about but not tried myself:
a) foam head (art supply or beauty supply stores, they cost under $5.00)
b) glass head (Pier 1 Imports stores, $15)
c) stuff the hat with plastic shopping bags or small towels until it's the right size
d) wear the wet hat on your head (!)
e) professional foam/wood/plastic hat blockers (not cheap)
A LYS sells pricey cylinders with rounded tops for hat blocking. Guess what? They look a lot like a bowl on top of a canister.