You can use T-Nuts they fit and work well....
I use that aluminum from Lowes with 10-32 bolts and nothing has slipped yet. I dropped my commander one day and the bolts held fine even when the dashboard brokes I had made out of wood as a prototype.
The slide Trax nuts that came with the commander are Brass.
Perfect-o. Nice job. Looks like you used a drill press or your better at hitting the center than I am. I think the 1/4-20's pulled because I was putting a side load on them as well as pulling. The ones that pulled were the ones holding the Fishing Buddy bracket I fashioned.
I do echo David's comment on why not be able to get them from W.S. without paying an arm or a leg. The last time I looked they were 4 for $20.00 and thats just is a bit greedy.
I only use a hand drill and a tap made like a screw driver. I cut the strips 1" long Then you draw a pencil line from corner. I use a center punch and make a dent right where the lines cross. Drill your hole and use the tap. Then it's perfectly in the center. :)
Then round the corners with a file, grinder or what ever. It pretty simple and low tech for tools. Most anyone should be able to do it.
I would prefer to use the stainless like Jeremy has, and you can go to the metal dealer and buy the stock. If you use stainless get some oil and use it when drilling and especially using the tap. WS are made from a bass or bronze which would both be tougher than aluminum. I have yet to have an issue with these anchors pulling out. Remember the entire trax is screwed down to a plastic kayak. LOL There has to be a limit to the amount of stress you apply to it just for that reason.