This is how I made my board with vacuum infusion:
From top to bottom:
carbon (10 cm wide strips)
Airex core 5-10 mm (two 5 mm sheets bonded with epoxy and microspheres)
glass (going over the edge to the top side)
Aramid (going over the edge to the top side)
Around the core and fibers:
There was no flow media, which would have made the process faster.
The epoxy was SP: Prime 20LV Epoxy Infusion System with Slow hardener.
It is really runny and slow curing.
I drilled 1,5 mm holes in 3 cm intervals from tip to tail to make the epoxy go to the base side. It seemed to work ok. Resin inlet is in the center, vacuum is from the sides.
Curing was done in sauna: 5h in 70-90C and board was left there over night. Board was still attached to the rocker table.
I didn't prepare the Airex pvc foam core in any way before the infusion process. It is a bit porous with closed cells. I haven't worked with balsa, but I don't think you need to prepare it. Maybe sanding with rough paper along the infusion direction might help.
The shop I bought the the materials was out of flow media. I did some testing on the resin flow before doing the board. Small grooves (~1mm*1mm) enhanced the resin flow a lot, but I ended up not to do that. I also bought some mosquito net which I think might have worked in helping the infusion but I didn't use that either. If the epoxy wasn't so runny, the infusion process wouldn't have worked without flow media.