I stock mostly velcro opening panels for seat sizes 17" and 18" but can order the others and do have a couple of the closed panels available.
I prefer the velcro opening panels because that makes it easy to shim for rider balance, but the closed panels do have an neat look.
Most people use the standard panel thickness which is a thin layer of formo(a stiff material), a layer of 2 or 2.5cm foam with excellent weight distribution and response-time properties which remain at all operation temps, and a single layer of a softer, shape-forming foam.
The thin panels don't have any of the soft foam and are best for riders under 120lbs or who prefer a bareback pad feel (and reduce stirrup use accordingly) or who use a fairly substantial treeless pad and need to reduce thickness.
The thick panels have a second layer of the soft foam and are recommended for riders over 185lbs or those over 175 competing in endurance.
It's possible to modify the panel inserts with the velcro opening panels, not with the closed panels so choose accordingly.