For background: I have some sci-fi technology based on sci-fi physics which depends on some fictional forces that interact differently with up quarks vs. down quarks. So, to get certain types of effects, you want to use materials with lots more down quarks than up quarks, and to get other types of effects, you want materials with an excess of up quarks over down quarks. In practice, that means that sometimes you want materials that are neutron rich (to get an excess of down quarks), and sometimes you want materials that are proton rich (to get an excess of up quarks), because Delta- and Delta++ aren't suitable for building stable matter out of.
Now, getting high neutron excesses in small spaces isn't too hard--elements get more neutron rich as they get heavier, so a block of U-238 is just about ideal**
Getting proton excess, however, is a little trickier. The ideal material should be hydrogen--it's nothing but protons. Except, hydrogen gas isn't very dense, and neither is liquid hydrogen, so the total number of protons you can pack into a given space in the form of liquid hydrogen is considerably lower than the total number of unpaired neutrons you can pack into the same space in the form of uranium.
So, what is the best way to get a really large excess of protons packed really densely? Materials that are solid, and furthermore materials with high tensile strength, are preferred, but if necessary this can be simulated with high-proton liquids or powders embedded in a matrix of less-protonaceous-but-stronger materials.
**Actually, you also want materials that are electrically non-conductive, so actually complex uranium ceramics are most ideal, but that's relatively less important.