I had a portable garage for a time. I got it with the optional extended height. Each of the hoops received an extra piece swedged to fit in the bottoms of the main hoops, had provisions to attach the rail along the bottoms, and for the diagonal bracing, too. Of course, the tarp and doors (end curtains) were sewn longer to suit, as well.
I would attempt to contact the OEM and see if they have an option package that you can buy. Depending on your climate, keeping your existing tarp/canvas with an open air bottom could actually be a benefit, allowing good ventilation while still maintaining a roof over your head.
Next option would be to build knee walls along both sides, say out of 2x6's, tie them together across the bottom with 2x4's so that they can't topple over, and anchor the whole ten assembly on top of them. Use pressure treated for the bottom sill of the knee walls for a temporary solution.
More permanent solution? Dig two trenches down a bit, level, tamp gravel, and build up CMU block knee walls to set your tent garage upon.
I always regretted not laying a good gravel base (over a sand covered moisture barrier) under my temp carport. The ground moisture was drawn up by the solar effect on the tent and I think the gravel base would have helped prevent this.
$.02