I built these some time ago to fit the etrack. I can put 8 MX bikes in a 18 foot trailer with plenty of room left for gear. The only bad part is that they come out so easy that your friends will still want you to help with there moving needs. If you own an enclosed trailer you know what I mean!
JFAB
I like this, great idea and you can also hook your tie down straps in the same e trac. I use something almost the same, and can get 10 bikes in a 20' trailer. Since you built you loops for the front wheel you could make them any way you wanted, I assume you hall 3 across, i would move the middle loop back abour 4 inches so the handlebars clear the outside bikes.
I would not put something in, that sticks above the floor that I could not take out (trip hazard), on one of my smaller trailers I built channel iron that fit into a bucket and when down they were basically flush with the floor, this was about 20 years ago, now you can buy them, I think somebody did the R & D thing (rob and duplicate)