Re: corner reflector
If the first surface the laser touches is simply a good quality white TiO2 paint, then IIRC, around 95% of the incoming radiation is reflected, albeit not back at the target. So 30 KW of light delivered would mean that about 1.5 KW needed to be dispersed. So, if you simply filled a suitable ablative matrix with TiO2 powder, then even without a dispersive effect of any smoke you're sorted. e.g. a phenolic resin matrix can do 200KJ/Kg, assuming a rather slow 10 seconds of attack at 1.5KW we have 15KJ to dissipate, so that hot patch needs to weigh 130g. Assuming that's about a 200mm hole, and assuming you can get away with 50:50 resin to filler whilst maintaining reflectivity- I think you might get away with about a 6mm thick panel and you might not even need to repaint the the car's bodywork after the attack (if you leave an air gap) (assuming I haven't messed up the calculation!).
I can't help feeling that researching how to make such lasers is a good idea even without a good end application- but surely this needs to improve by a couple of orders of magnitude to start to be a useful weapon compared to projectiles?