To maintain stability of the bike platform, it's imperative that you don't allow the Buckets to skirt outboard from each other (by the accumulating effect of small wiggling motion, IE walking).
To accomplish this task a simple 1 x 2 fixture serves this purpose brilliantly.
With everything positioned as perfectly as you can get it, measure the distance between the starboard and port Bucket lips and cut a 1 x2 to that size less 1/8".
This distance between Buckets will vary depending on the specific bike. The mountain Bike I have required a length after subtracting 1/8" came to be 7+3/4" in length.