(a) elected officials could hold only cash, bonds, and index funds, specifically bonds and index funds that are open the general public and have at least $XX assets, to prevent engineering an index fund available only one one or a few people.
(b) pay them better to make up for the loss of gravy. Sure, the pay raise would not be sufficient to make up for the lost income, but it should be high enough that none of them could complain they can't live on the salary. Paying the senate majority leader under $200K / year is not enough, considering they often have living expenses in Washington and their home state. Lower ranking members get paid less. Pay the leader $1M/year and everyone else $500K/year and then prohibit anything but vanilla investments.