I guess I’m bored so I’m overthinking I’m sure. If you want to go the cheapest route then reinforcing the current lid would be how I would do it. Just so it elevates any sag and will maybe get your latching feature back.
However the depth of tub being used is important. If it’s a shallow tub I would go the route of two UTH’s and probably insulating tub as mentioned and covering existing hole. The box with foam peanuts is a good idea.
If it’s a deep tub like 11”+ or 30+ cm the lamp would be my go too. Since it’s already in play. If there is no screen that should be addressed. There should be no way the snake could ever make contact with the lamp heat source. They get really hot. My CHE’s see temps over 200 degrees when on. Maybe with a pulse thermometer the lamp setup won’t go higher than setting but on/off type get hot.