1. A magnetic resonance imaging (MRI) phantom comprising an outer container comprising: a first portion comprising a first wall; a second portion opposingly disposed to the first portion and sealingly engaged to the first portion, the second portion comprising a second wall; and an internal volume bounded by the first wall and the second wall, the internal volume being hollow and configured to receive a fluid; a sample holder disposed in the internal volume of the outer container; a first sealing surface to form a seal in combination with the second portion; a first fill port distal to the first sealing surface; a first cover member disposed on the first portion to cover the first fill port, wherein the second portion further comprises: a second sealing surface to form the seal in combination with the first portion; and a second fill port distal to the second sealing surface; a second cover member disposed on the second portion to cover the second fill port, and a plurality of fasteners to engagingly fasten the first portion to the second portion and to immobilize the sample holder in the internal volume, wherein the MRI phantom is configured to maintain a constant temperature of the internal volume.