Hey I did this last year. I used celotex or whatever brand equivalent. I personally found it really easy to cut to size with a knife and then fit in between the joists. It seemed to have the best insulation rating.

I supported it with nails underneath knocked into the joists and then filled the gaps with insulation foam using a foam gun. The broad headed nails were cheaper than the custom supports or knocking in blocks of wood to support them.

Follow these best practices:

https://www.gov.uk/government/publications/insulating-suspended-timber-floors-best-practice

To prevent blocking anything and hopefully avoiding any damp problems in the future! They suggested quite deep insulation, i think i used 100 or 150mm but can't remember, whatever it suggested in that doc.

Hope that helps.