the best method I have found is - I took 1 inch wide elastic about 10 inches length. I attached a small square of ABS (1 x 1 inch) to the elastic with a rivet, on each end of the elastic. then E6000 glue the abs squares inside the calf pieces forming a U shapped "stirrup", so that the bottom of the U is just at the opening to the ankle. I put my leg into the stirrup while holding the back of the calf open, then slide into the boot. There is enough stretch and tension with the elastic to keep it down very tight.
I agree that your boots are not completely accurate, and would not qualify you for advanced status like EIB - but they should be fine for general 501st approval IMO.