In the U.S., wort is often "canned" for safe storage to be used for yeast starters without having to heat and cool a wort using expensive and difficult to store DME. We tend to use mason jars, a common item similar to the Weck-style jars in Europe. In order to inhibit botulism toxin formation, pressure and heat must be used to destroy any spores. Without a pressure cooker/canner it is impossible to know your wort is safe. Although the risks are low, it is worth investing the small amount in reasonable equipment to protect one's life. Here in Poland, borosilicate lab jars are affordable (labglas.pl has good prices). These jars are autoclavable, allowing them to be sterilized in pressurized cookers. They act as a great substitute for mason jars, and do not have metal components which can oxidize.
My 12-liter pressure cooker can fit 7 x 500 mL Labglas.pl autoclavable bottles without issue. I adapted a perforated flat false bottom for use in the cooker, keeping the jars off the bottom. The bottom of the cooker is filled with water to create the humidity necessary to sterilize.
When making a typical batch with wort suitable for starters, mashing more grain and using some of the runnings for this purpose is ideal. Starter wort should be between 7-10 BLG for optimal yeast propogation. Small mashes on the side can be a quick project for producing starter wort as well. In my setup, I am capable of running the pressure cooker next to my boil kettle, doing both at once.
Pre-boiling the starter wort helps drop out proteins in the hot break, but is not necessary. If you use the wort directly from the mash, it will have more sediment. With these bottles and their neck (unlike Mason jars) it is easy to keep most of the sediment in the bottle when pouring into your flask or other starter container.
I use a funnel to fill the bottles. If you fill them to the top, they will boil over during pressure cooking. Even at these kinds of levels, you will lose some wort in some cases. If you attempt to depressurize the cooker before it has cooled down completely, the change in pressure will cause them to boil over more than usual. It is best to make a batch matching the size of your pressure cooker, and let it sit overnight. The lids should be placed on and turned partially so that they are not fully sealed, but stay on the bottle.
Once they are cool, open the cooker and tighten down the lids. Wash the jars off and store them. I open 2-3 and dump them into a sanitized flask and pitch the yeast directly.