You can mod the hold of a freighter to hold that much, an examble is below.
See
bold area. Use FLMM to open the data file/ships/shiparch and find the ship you want to mod.
[Ship
ids_name = 237035
ids_info = 66573
ids_info1 = 66574
ids_info2 = 66608
ids_info3 = 66575
ship_class = 2
nickname = li_freighter
LODranges = 0, 80, 120, 200, 300, 1200
msg_id_prefix = gcs_refer_shiparch_Libf
mission_property = can_use_berths
type = FREIGHTER
DA_archetype = ships\liberty\li_freighter\li_freighter.cmp
material_library = ships\liberty\li_playerships.mat
material_library = fx\envmapbasic.mat
envmap_material = envmapbasic
cockpit = cockpits\liberty\l_freighter.ini
pilot_mesh = generic_pilot
nanobot_limit = 12
shield_battery_limit = 12
mass = 300.000000
linear_drag = 1.000000
hold_size = 80 ; change this to 20000 or what ever you want.
fuse = intermed_damage_smallship01, 0.000000, 400
fuse = intermed_damage_smallship02, 0.000000, 200
fuse = intermed_damage_smallship03, 0.000000, 100
max_bank_angle = 15
camera_offset = 8, 45
camera_angular_acceleration = 0.035000
camera_horizontal_turn_angle = 20
camera_vertical_turn_up_angle = 5
camera_vertical_turn_down_angle = 20
camera_turn_look_ahead_slerp_amount = 1.000000
hit_pts = 1600
explosion_arch = explosion_li_freighter
surface_hit_effects = 0, small_hull_hit_light01, small_hull_hit_light02, small_hull_hit_light03
surface_hit_effects = 150, small_hull_hit_medium01, small_hull_hit_medium02, small_hull_hit_medium03
surface_hit_effects = 300, small_hull_hit_heavy01, small_hull_hit_heavy02, small_hull_hit_heavy03
nudge_force = 30000.000000
strafe_force = 20000
strafe_power_usage = 2
bay_door_anim = Sc_open baydoor
bay_doors_open_snd = cargo_doors_open
bay_doors_close_snd = cargo_doors_close
HP_bay_surface = HpBayDoor01
HP_bay_external = HpBayDoor02
HP_tractor_source = HpTractor_Source
num_exhaust_nozzles = 2
shield_link = l_freighter_shield01, HpMount, HpShield01
hp_type = hp_gun_special_3, HpWeapon01, HpWeapon02, HpWeapon03
hp_type = hp_gun_special_2, HpWeapon01, HpWeapon02, HpWeapon03
hp_type = hp_gun_special_1, HpWeapon01, HpWeapon02, HpWeapon03
hp_type = hp_freighter_shield_special_3, HpShield01
hp_type = hp_freighter_shield_special_2, HpShield01
hp_type = hp_freighter_shield_special_1, HpShield01
hp_type = hp_thruster, HpThruster01
hp_type = hp_mine_dropper, HpMine01
hp_type = hp_countermeasure_dropper, HpCM01
hp_type = hp_turret_special_3, HpTurret01, HpTurret02, HpTurret03, HpTurret04, HpTurret05
hp_type = hp_turret_special_2, HpTurret01, HpTurret02, HpTurret03, HpTurret04, HpTurret05
hp_type = hp_turret_special_1, HpTurret01, HpTurret02, HpTurret03, HpTurret04, HpTurret05
steering_torque = 54000.000000, 54000.000000, 104000.000000
angular_drag = 60000.000000, 60000.000000, 60000.000000
rotation_inertia = 16800.000000, 16800.000000, 16800.000000