Since you’re tenant, I’d not give too too much of a fuck. Measure the dimensions of the square shaped hole & grab some mdf. It’s the mildly cheapo stuff cabinets are made out of lately. You should plan to cut into two pieces so you can fit the upper and lower around the pipes. If you cut it right it should shimmy in and you can glue it. If maintenance needs to get in there they’ll just cut it open again.