I wouldn't weld it and I wouldn't use HDPE. it's tough to get good welds and requires considerable experience. You really don't want a bash plate to break up and there are cheaper and much easier ways to get a plate that will be a lot stronger. if you do go with sheet stock and welding don't get welding rod. Use thin strips cut from the base material you are using. the resins must match exactly and this will do it. Just cut strips that match your welders guide port. Forget a nitrogen rig or even a compressed air gun.. $$$$. A simpler "heatgun" type welder will work fine.It's hotter than an ordinary heatgun and has a special tip. I have one. it works OK but I get more mileage with Fiberglass, including for repairing thermoplastics.
I would also not use HDPE. Too soft. UHMW-PE would be better. (HDPE=High Density polyethelene. UHMW-PE = Ultra High Molecular Weight Polyethelene). The UHMW is more abrasion resistant. But with either one if you hit a rough rock the stone will dig in and you may pitch forward more than sliding over.
I second the fiberglass. Easier to do. An exact fit every time. Easy to maintain as it gets bashed up, fantastically strong, very hard, will slip over things like a metal plate would (althou it will get scraped up. I made a large fiberglass part once, the size of the back end of a small car. It ended up being intentionally run over with an extremely large bulldozer. Didn't even crack (to my dismay, I wanted to destroy it).
You can make a styrofoam form right on the bike and then glass over that and right over your frame tubes as well, then pop the peice off, trim it and get the form out, throw some paint on it and then bolt it up the same way you would a metal plate. it will fit perfectly on the frame tubes and at about 1/4" thick, will take anything you can do to it. You will be using epoxy resin and it has a good shelf life. Easy to sand you plate and ad a layer or three as it gets beat up.
I can go into some more detail about the construction if you are interested. I'm starting a project to make some FG ram air ducts for my RS and I'll probably do a post on that in the coming months.