I made a rectangular (13"x9") tower of 4x4 lumber, screwed and glued (construction adhesive) together. It is 3 'layers' tall (~11") overall. The space between the layers allows a 2x4 or 4x4 to pass through.
With a 2x4 placed in between the top layers it is ~7" high, or the 2x4 can be placed across the top for ~12 1/2" height, or a 4x4 can be placed across the top for ~14 1/2" height. We take it with us and have used it frequently.
Sherm & Terry w/rescue Eydie (min Schnauzer) & Charley (std Poodle)
2015 Jay Flight 27RLS, GY Endurance (E), Days: 102 '15, 90 '16, 80 '17, 161 '18
2006 Ford F350 PSD, 4WD, CC, LB, SRW, Cmpr. pkg, Equal-i-zer 4-p, ContiTrac TR
Full timing: Some will think you're crazy, some will be envious, just enjoy the freedom!