Not everyone has the same perception of comfort...
For some people, if the foot is not completely compressed and the pain is bearable for more than 15 minutes, the footpocket is too loose and you will lose propulsion !
For others, a simple rubbing will certainly lead to blisters.
The skin of the foot, when it is in the water, can be easily altered and with repeated training, it is difficult to regenerate. In any type of fin footpocket, there will always be some shafing, but the effects can be limited
You can use anti-shafing creams, but with water, their effect dissipates after a while.
On the other hand, never use Vaseline, it deteriorates the rubber which will become like cardboard.
In the same way, some people recommend coating the shoe with Vaseline if it is not to be used for a certain period of time, especially in hot and humid regions. This treatment will deteriorate your fins. It is best to store your fins in a box with desiccant bags.
The best is to cover the foot so that it is not in direct contact with the rubber.
Some people use a simple plastic bag and this is very effective !
You can also use a thin sock, either cotton or lycra.
Finally, you can use a neoprene sock of 1, 2, 3 ... or 8 mm !
Depending on whether you go into the water in the morning or the afternoon (the foot is more swollen), the temperature of the water and the length of immersion, the volume of the foot will change. It's a good idea to have several socks of different thicknesses so that the footpocket always fits properly.
With the open-heel custom-made footpocket, you may also experience friction on the Achilles tendon. To improve comfort, you can sew a piece of neoprene around the strap. Your fins will be much more comfortable.