Yep. Problem with HDPE is that it is almost impossible to glue it on.
I mounted my tinny on an off road trailer that shares wheels with the motorhome. That way there is lots of space for other stuff inside and there are no restrictions on where it can go And it tilts to make loading and unloading easy.
I used 100mm (width) x 25mm (thickness) strips, the length of the subframe, scarfing joints where needed. Drilled and tapped the subframe at 300mm centers. Counter-bored the HDPE to 18mm depth, leaving 7mm of material for fastening. With the bolt and washer in place, this results in an approx. 12mm wear before the bolt heads hit the chassis. I also counter-bored for the chassis rivets.
By the looks of your subframe main rails, you could use 50 x 25 strips, and in lieu of drill and tap, counter-bore the mounting holes (just large enough for the drive bit), use 14g metal tec screws secured with sikaflex (these have a 3/8 drive head) plug the counter bore with sikaflex.
If I need to replace it at a later stage, I can use the old strips as a template.
The last few days I have been chasing up more appropriate skid material. I have ordered some HDPE to replace the neoprene. It will be bolted to the subframe as recommended.
Meanwhile I have drilled and tapped the service plugs for applying rust preventative into the cross members. I'm still yet to decide on what fluid would be best for the task. Fish oil or lanoltec come to mind first, but I'm open to suggestions
Also I finally replaced my phone screen and was able to upload this vid that we flimed a while back.
I also used 25mm thick HDPE for the subframe skid material, used 12g self tapping pan head gal screws to fasten to subframe. Countersunk the HDPE just enough for screw clearance plus a couple mm- wanted as much meat as possible under the screws, Just butted the HDPE together, the subframe doesn't walk around only flex's minimally for most of it's life so wear is not really an issue, my HDPE supplier said it would outlast the truck.
A bit late now as you have it all welded but I coated the internals with corrosion preventative before welding the side rails on, used a sponge and some fencing wire to make a pull thru. That way could ensure good coverage, not sure how you will go thru those small holes unless you use lots of fluid. I did it for piece of mind but the commercial blokes don't bother so probably not needed at all. I use an aviation product called Corban, only because I had a drum of it on hand.
Just one of the many things we have to decide on with these builds, Good luck and have fun.
My opinion only but i like the fishoil. If you have a compressor get a engine cleaning gun and spray it into all the hollow sections at a high preasure and it will mist into every nock and cranny. Fish oil will eventualy dry but stay sticky so water will not float it off like some oil does. Have used it in boat trailer channels and it works well even with salt water dunking