If one doesn't have a kippah (or anything to cover his head) should he eat without making a bracha or not make one at all?
Yes he should make a bracha, but he should cover his head with his hand. [If he has any random material e.g. sleeve, that would be better.]
1 - There are opinions that say that one's own hand would be a good covering, and the Mishna Berura (2:12) writes that in case of need some rely on this opinion. Even though it would be better to cover the head with a sleeve or other material.
2 - The Pri Megadim (פתיחה להלכות ברכות ס"ק ו) writes that בדיעבד - after the fact, a bracha made without a covering is valid. We also know the rule that שעת הדחק כבדיעבד דמי, the time of pressure is equivilent to after the fact (ט"ז יו"ד סי' צ"א ס"ק ג). Where no head covering is available, even a random material, it would be like בדיעבד and one can make the bracha even if the hand is not considered a valid choice.