I have a free standing Condor wheel chock. As described by Sundancer a small metal bracket is permanently bolted to the floor. You slide the Condor into the slots on the bracket and then a set screw keeps it from backing out.
If hauling two bikes you will want to stagger the brackets by a few inches so you don't end up handle bar to handlebar. Plus you will still want to strap the bikes down.
When the bikes are removed and you want to use the garage you can remove the Condors and only the brackets remain.
Condor also sells a ETRAC version and a Trailer only version without outrigger arms.
You will have to get through the Mesh or Coroplast sealed underbelly to get to the nuts.. I have the Mesh sealed underbelly membrane. I cut a small slit and fastened the bolts then sealed the slits with 4" Eternabond tape. I only had some white so a little black paint and you can hardly notice. Eternabond does sell black tape.
I was just under there the other day and it has sealed really well.