Pressure treated or outdoor lumber. Fasten a 4X4 beam across both sets of bed mounts with carriage bolts so they are running from side to side. Use those as your foundation and mount 2X6’s perpendicular to them. You can construct removable sideboards that slide in and out of U brackets that can be purchased at Ace hardware.
Pressure treated 2 x 4’s, 2 x 6’s…oak, if you can get it…
Pressure treated or outdoor lumber. Fasten a 4X4 beam across both sets of bed mounts with carriage bolts so they are running from side to side. Use those as your foundation and mount 2X6’s perpendicular to them. You can construct removable sideboards that slide in and out of U brackets that can be purchased at Ace hardware.