I got a folding cane to take care of this issue. Folded or partially folded it fits nicely in the ECV basket or back of seat pouch. Then when I needed, I would just pull it out and it snapped itself together and away I would go.
I would ask the company that you are getting the ECV from to add one to your reservation. I would also bring some small bungie cords to help in case they don't stay in the clips. I have one on the back of my scooter.
FYI. There are 2 different types of holders. One holds they typical underarm crutches and there is another one that hold forearm crutches. You might want to specify which one you need.
Use bungy cords, they can stand straight up behind your seat and loop the bungy cords crisscrossed thru the crutches, attach the hooks to the arm rest hinges.