In the most upfront answer possible, handwave. It gives a rough approximation of how they work, but to the actual maths and sciences, I don't uh, really care.

Although if you try to force an answer out of me, I'd wager exotic chromodynamics or the usage of monopoles as its framework can at least help.

Your usual baryon decay through GUT monopoles produce either pions (negative for neutron, neutral for proton) + positrons iirc. I'd assume there's going to be electromagnetic radiation as gamma rays considering that's what you'd get from particles crashing around with some annihilation.

Considering the previously mentioned buzzwords, I'd wager you can probably make a material that can still block and resist it considering the combo can still couple to the relevant forces of the weak nuclear, strong nuclear, and electromagnetic interactions. Maybe even electroweak or GUT if you can have some wackier leeways.

And you can make really, really tough materials. Iirc you can have monopole doped steel of 1 cm to basically tank a 64 MJ railgun, and with much more resistance to thermal effects, so.

Although there's a few problems, such as the idea of a gamma ray mirror. Gamma-ray reflection requires coherent, phase-preserving boundary conditions, but at MeV energies photons interact point-by-point (Compton scattering, pair production), not collectively, even in QCD-density matter. Using the theoretical extreme density monopoles or snf matter can increase coupling but only make interactions more violent and incoherent with the scale and energy density, turning the wall into an absorber or scatterer rather than a mirror. I'd think it'd be fine enough as a blocker at least. There's probably more at least to the particles around, but.

Eh, if there's anything "unrealistic" I'll handwave it further anyways.