Proper size of Portuguese custard tart

I'm pretty sure it's the famous Sao Jorge cheese from the Azores although there are many cheeses from these islands:

There's some good information here:

http://www.culturecheesemag.com/Sao%20Jorge

The Azores have traditionally been home to a variety of cheeses. These range from fresher styles, designed to be eaten within a few days of production, to more robust, aged cheeses able to withstand the rigors of transportation - and consequently often sold to ships’ crews to sustain them for many months at sea.

Made from raw cow’s milk, São Jorge cheese falls into this second category. It is the largest of the Portuguese cheeses weighing between 8kg (16 lb) to 12kg (26lb) and is aged for between three and seven months before release.

To make the cheese, cows are milked twice a day and the evening milk gets delivered to the cheese plant about 8:30pm with cheesemaking commencing right away, continuing through the night from about 9 pm to 4 am. Then, after morning milking another round of cheese is made.

A cooked curd pressed cheese, São Jorge is made by three dairies on the island although just one produces cheese that has been awarded DOP (name protected) status. The DOP status is awarded to cheeses matured for a minimum of 90 days although usually wheels of São Jorge are released at three, four and seven months.

Short Answer: Probably, but it won't taste as good.

Longer Answer: Given that most packaged custards (if you mean the powdered sort such as Bird's or Jello) do not contain eggs, you might find that the custard does not behave quite the same and certainly will not taste as rich. They tend to use yellow food colouring to give the bright yellow colour that you would get from the egg yolks. It's a bit like asking if you can substitute Cool Whip for Whipped Cream. Yes, but....