Holster for G19 MOS with light. Must have retention AND cover for optic. Anything?

[ClangClang]

I'm looking for a highly specialized holster and having no luck finding anything. It will be used for Run N Gun competitions (Basically a biathlon in the woods. 3-8 miles of running, obstacles, streams to ford, tunnels to crawl through, with anywhere from 5-10 multigun stages).

I need a holster that has active retention to keep the pistol in place for 2 hours of running/rolling/crawling and also preferably some sort of full coverage to protect the optic and the action from getting covered in mud and/or damaged. And finally, beggars WILL be choosers, because I would like for it to able to accommodate a TLR-1 light.

I'm thinking a nylon surplus holster will be best. The draw shouldn't be cumbersome, but it does not need to be a USPSA style race holster draw. And if the retention is some sort of buckled flap, I can disengage the flap as I arrive at the stage before the shot clock starts. So I have some flexibility there.

At this point, I'm thinking my best option is just get a standard Safariland ALS and then craft a "shower cap" out of waterproof cordura and elastic to fit over/around the entire holster. I can just pull the cover off the holster as I'm running up to the stage before I get to the line. Any better ideas? HBARTactical, you have any suggestions for something custom?