I’ve got some 10lb fluoro laying around that I’m not going to use for anything. Could I put a couple layers of it on a reel as backing to prevent the risk of braid slipping, or to I have to use mono?

Yes you can use fluoro 

I use either. 

I'd save the expensive flouro for leader material on braid-loaded reels.

 

This is what I use for backing

Casting reels - $10.99 for 1500yds

 

Spinning reels - $6.49 for 3000yds

You can use anything.  I’ve even used tape to build up the spool.

@Jig Man I use electrical tape on my spinning reels before I put the backing of mono on to give that extra bite on thr spool.

Use cheapest stuff you got.  I ordered a 900 yard spool of no name for my backing 

I find it absurd (even though I do it) that spools hold enough line that we use backing to save from spooling on $$$ line. Wouldn’t shallower spools simply solve that?

32 minutes ago, BrianMDTX said:

I find it absurd (even though I do it) that spools hold enough line that we use backing to save from spooling on $$$ line. Wouldn’t shallower spools simply solve that?

I deliberately purchase deep spools (casting & spinning) and use very little backing.

Often less than 1/3 of the spool is backing, leaving quite a bit room left for my mainline.

Even then 'the backing' is still something decent and equivalent in size & test to whatever the mainline is.

Clearly, a 2 lb bass or even 6 lb bass would never spin off enough line

against the drag to be of any concern.

However, there are some other rather plus sized fish here besides bass

that can and will completely dump a shallow spool reel in a mater of seconds.

It's a helpless feeling.

So I come prepared. 

A-Jay