Maybe that strange looking plasticy looking substance is solid- and maybe it is atatched to the truss in some way. That would give the support you ask for and you could still keep the 5mm holes.
I found a "lowrider thingy" that you mount on the luggage block also. Allowed to use two paniers in the front, but since it is mounted on the luggageblock there is a limit to how much you can load on it. Was not sure if it was interesting so I did not keep the link. Now I must look back..
@ ilynne: Take some time, take a deep breath and do some research. There is plenty good DIY solutions out there. If you want that frame I suggest you find a different way to mount it than the luggagetruss.