Ok guys I have a Lg/XL Eagle PC that I have the soft armor backers in. I have an XL set of lvl III icw ceramic plates  that fit perfect. I also have a set of Large Ti stand alone plates that swim quite a bit in the PC.

I personally perfer the Ti plates because they are quite a bit thinner and alot loess bulky, but I don't like the fact they swim around a bit in the PC. I obviously want them to stay in their proper place to afford the protection they are designed for.

Is there an easy solution to keeping the Large plates in place.

tag, i've got this issue too.. are they the Ti plates from DSG?

Yeah.

Dual sided adhesive velcro : The widest you can find. Put 4 strips on the back of the plates horizontal. I did that with a set of Paca's I used to have in my Eagle that kept shifting.

you can use some stiff foam to keep it in there tight

I had this problem in the older Safariland vests.  I took a foam ISO mat and cut strips and duct taped them to the edges to fill the gaps and keep it in place.  I tried velcro at first but it started to peel the surface of the plate away.