I kinda like the BHI Enhanced STRIKE Recon Chest Harness. Their regular STRIKE RCH is basically a knockoff of the RRV. You can attach a rear plate carrier from the STRIKE Gen4 Plate Carrier (BHI's take on the Allied Industries MBSS) to get a full plate setup. You can use it in conjunction with a PACA, Marine Force Recon style, if you want soft armor.
Another option is the SOTech Callahan. The actual setup is a plate carrier with a MOLLE chest harness that can be integrated (they can also be worn alone, separately).
Depending on what you plan on mounting on it, you could also just use mag shingles; for small things like pistol mags or a radio they're fine.
As for going prone comfortably... that's pretty debatable. If you're using hard plates, there's going to be a level of awkwardness, but it will also prevent mags from digging into your ribs and abdomen. If getting and staying small is really a critical issue (greensiding) you may want to travel lighter, move mags to a chest rig and maybe run a MOLLE belt.
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