snow peak lid

One way is to cut out a disk of oven liner. Heat it over an alcohol stove (or very carefully over any stove) until it shows red in dim light. Quinch it in cold water. Play with this procedure until it is buttery soft. Then simply mash it into whatever shape you want by rubbing it with any smooth tool over a form (the pot you will use is the ideal form). Popsicle sticks, hardwoor dowels, plastic handles all make good ad hoc rubbing tools for this job. If you want the lid to fit inside the pot, center it over the mouth of the pot and rub it into a smooth shape, then trim any excess that extends beyond the pot rim. A fold of aluminum repair tape or flu tape makes a perfect 'knob' for such a lid. One or three holes beside one edge will let steam escape without burning you when you reach for the knob.

You can simply crimp the edges around the lip of the pot to fit as tight or as loose as you want:

FYI, If you're not cooking with the SP 450, a plastic coffee-cup lid fits it well to avoid spills: