1985 eldo convertible - child seat suggestions

we are now flirting with spring finally in north jersey. im looking forward to cruising top down with the family soon. i have a 3 and 5 yr old and need to get them in the rear seats. the 5yr old is in a full booster, but the 3yr old is stil on a full strap in seat. how can i make this work with the lap belts in the back seats? i remember the days when we just sat back there with the lap belts on and thats it , but now i would get thrown in prison