I filled those hole on my elky about three years ago using a mig gasless welder with .030 wire, I had to hammer them in first because someone tightened the screws so tight it bent the metal out around the holes, then i slowly made a few tacks around the hole going in a circle letting it cool for a few seconds between each tack, then when the hole got smaller after going around a few times i just hit with one long weld to melt it all together, then i just spent allot of time with the angle grinder and the sander to get it all smooth because i didn't want to use any body filler, if you take your time you wont warp the metal and you wont need any body filler. my only issue was there is no access to the back to primer the welds but after three years no rust yet but its only a matter of time, but some day i will replace the inner bed panels and i will address this issue, if not it will give me something to fix many many years down the road if it ever rusted through.