Help with GRP repair

Hello all, I am new to the forum and my need is for a car bumper repair (my boat is fine!).

I really need the advice of someone who works with GRP. I can glass to a basic standard but I am having a problem with a GRP bmumper I am modifying for my MGB.
I have attached a pic to show what I am attempting to do. I am trying to compeletly fill two recessed light holes in the bumper. I have layered the back of each recess but am left with 20mm ish depth to fill in. If I keep laying up with just mat it will take forever!

My question is; What could I add behind my next layers of mat to pack out the depth so i can then surface fill?
I appreciate anyones help with this especially as this is a boat site!

Thank you.

 

You might want to fill with wood, ridgid foam insulation, etc which is inbedded in thicked resin, just shy of the surface, then you can fiberglass a layer on top to close it, hope you are good at fairing

 

You could put just about anything in there, foam, wood, body filler, an old used diaper, it just needs to hold up so you can put more glass over it.

 

I'd use foam,because I have some,you may not find it particularly easy to get hold of small quantities of foam so as mentioned,alternatives may be necessary.Don't be tempted to try polystyrene because the most common type is likely to be attacked by polyester resin.Wood should be safe and can be bedded in a runny mix of resin and a convenient filler powder.For such small areas it shouldn't be a big deal to arrive at a nice outer surface.

 

Bondo or other light weight body filler would be my choice.

 

Small quantities of polyester compatible foam usually come from a can. Now for this I wouldn't go buy a new can, I would just take a cardboard box, fold the edges in such a way that it sits flush in the hole and laminate over it.