I believe there is one thing OP can do to incentivize people to continue re-investing during the round.

Currently, he starts the payouts approximately 15-20 mins after the first deposit. This means people have already deposited 70 BTC, and the people who are paid out do not want to re-invest as there is already 70 BTC.

BUT, if the payments were instant like the Fees this might change. OP is instantly transferring the fees to his fee address, but payouts wait 20 mins. Making the payments also instant would give people who are paid to reinvest and therefore the active duration of the round would be much longer.

Just a thought...