My thoughts on an approach.

offer odds at the tote or worse odds early on the top 5 selections. If any get taken then adjust your other prices slightly upwards to get matched.

If using software this is quite easy and you could offer odds on all selections and make adjustments easily and quickly. You will not be able to make a book without the right software because it will send through a heap of bets at once to green up a lot quicker then you could do it using the web front end.